Animal found in decreasing numbers (3)
I believe the answer is:
gnu
'animal' is the definition.
(I've seen this before)
'in decreasing numbers' is the wordplay.
'in' indicates the answer is hidden within the clue.
'GNU' is hidden within 'decreasing numbers'.
'found' acts as a link.
(Other definitions for gnu that I've seen before include "Literally wild beast" , "Wildebeest" , "Animal" , "Large heavy antelope" , "Gun for a kind of deer" .)