Police seizing unknown pet apes (8)
I believe the answer is:
copycats
'apes' is the definition.
('copycat' can be a synonym of 'ape')
'police seizing unknown pet' is the wordplay.
'police' becomes 'cops' (I've seen this in another clue).
'seizing' indicates putting letters inside.
'unknown' becomes 'y' (algebra).
'pet' becomes 'cat' (I have seen 'Common pet ' mean 'cat' so perhaps 'pet' could also mean 'cat').
'y'+'cat'='ycat'
'cops' going around 'ycat' is 'COPYCATS'.
(Other definitions for copycats that I've seen before include "they aren't the genuine article" , "Feline kind of imitators" .)