The phrase "a clear decline in" is correct and usable in written English.
It can be used when discussing a noticeable decrease in a particular metric, trend, or condition.
Example: "The report indicated a clear decline in sales over the last quarter."
Alternatives: "a significant drop in" or "a noticeable decrease in".
Since the 1960s the nutrient load of Lake Balaton has increased significantly, causing a clear decline in water quality.
Research by the Early Warning Project for example, has shown a clear decline in mass killings in wars and conflicts since 1992.
Between 2004 and 2010 there was a clear decline in the proportion of the public who rated standards of conduct as either 'quite high' or 'very high'.
There is generally a close correlation between annual precipitation and elevation, and there is a clear decline in annual rainfall toward the northwest.
Children from homes that were demolished showed a clear decline in mental health, with classic signs of trauma: withdrawal, depression and anxiety.
Even so, the Minnesota Internet Traffic Studies (MINTS) project, which Dr Odlyzko heads, shows a clear decline in traffic growth compared with previous years.
There is a clearer decline in diagnostic performance (i.e., proportion of the infected population detected) in low transmission settings for a threshold of 100 parasites per µl compared with 10 or 1 parasite per µl.
A clear decline was evident: In 2005, 19.8 percent of wounded American soldiers died from their injuries.
According to our longitudinal data, the trajectory of PS from 3 months to 24 months postpartum revealed a clear decline over time in Ghana.
That's a decent result for Mr. Obama relative to their usual take on the race, since the poll is somewhat Republican-leaning, but also suggests a clear decline from his peak in the poll a couple of days ago.
For men, a clear cyclic pattern in employment rates can be seen, corresponding to the recessions of the early 1980s and early 1990s; a clear decline is also evident in 2009.
