Decline evident finally: time flies (7)
I believe the answer is:
diptera
'flies' is the definition.
(I've seen this before)
'decline evident finally time' is the wordplay.
'decline' becomes 'dip' (both can mean to fall or droop).
'finally' says to take the final letters.
'time' becomes 'era' (era is a kind of time).
The last letter of 'evident' is 't'.
'dip'+'t'+'era'='DIPTERA'
(Other definitions for diptera that I've seen before include "Flighty little types" , "Insects" , "Flies" .)