"if partly" is correct and usable in written English.
It is used to mean "if partially" or "to some extent." For example, "If partly responsible for the accident, I should accept responsibility."
But it's a powerful book, even if partly fictional.
Although the novel is fanciful (if partly prophetic), it has its roots in Broun's personal experiences.
If partly hydrogenated oil is an ingredient, there is some trans fat in the food.
But a few were following Mr. Markowitz's lead and making overtures toward the new hometown queen, even if partly in jest.
Moments like this form a voiceover that give the film a colourful, literary ooze, as if partly written by Billy himself.
If partly assessed as an Olympic tourist tax, this money could help mitigate the significant ecological damages that even a smaller Olympics would have.
The pros: IF will result in weight loss, if not partly because you just eat less.
Again, its costs if it loses are paid by us, as licence-fee payers, while our costs, even if we win, are partly – if not entirely if we lose – paid by us, along with their costs.
Their costs if they lose are paid by their shareholders, while our costs, even if we win, are partly – if not entirely if we don't – paid by us.
Even Mr Greenspan has recanted, if only partly.
Justice David H. Souter, whose seat Judge Sotomayor would fill if confirmed, partly dissented from that ruling.
