The phrase "a weakness then" is correct and usable in written English.
It can be used to refer to a specific weakness that has been identified or discussed previously in a conversation or text.
Example: "After analyzing the team's performance, we concluded that a weakness then was the lack of communication among members."
Alternatives: "a flaw at that time" or "a shortcoming in that moment".
Rocha plays a bullying, opportunistic game, keeping safe until he sees a weakness, then rushing in violently.
It's something of a weakness then that the choreographer Michael Clark, included as part of the Whitney Biennial 2012, cannot fully separate from his brash, younger self.
So if you say, "I sometimes take on too much", as a weakness then follow it up with an explanation of the way you manage that weakness.
He would never hurt anybody". Speaking of that side of his character, Chamberlain once had this to say: "If that's considered a weakness, then I'm guilty". Chamberlain was a lifelong bachelor who told the Associated Press in 1991 that "the women who I have been the most attracted to, the most in love with, I've pushed away the strongest.
If, in fact, doing any of them too well turned a strength into a weakness, then we should be seeing worse results for leaders who score in the 90th percentile and above on a trait than for those scoring well, but not overly well — say at the 65th percentile.
If Super has a weakness, then it's a surfeit of songs with scant or non-existent lyrics and a dearth of the ballads that are often the stealth highlights of Pet Shop Boys albums, aside from the Kraftwerkian Sad Robot World (not a metaphor for online culture; it's actually about robots).
If there is any weakness then a graft can be taken from the calf and put over the injury site to make it stronger.
If fame is a sign of weakness then secrecy is a sign of success.
As the old joke goes, if in your opinion, neither of these two colleagues has a really serious weakness, then the laws of statistics suggest that you do.
The king began with an attack on the Greco-Bactrian kingdom, which at the time was going through a period of weakness; then he turned against the west and declared himself independent of the Seleucids.
Asian weakness then took a toll on Europe and Japan, resulting in an unusually swift and sharp deceleration in the flow of global trade.
