A flower an animal found by mistake (7)
I believe the answer is:
cowslip
'a flower' is the definition.
(I've seen this before)
'an animal found by mistake' is the wordplay.
'an animal' becomes 'cow' (cow is a kind of animal).
'found by' says to put letters next to each other.
'mistake' becomes 'slip' (a slip is a blunder or mistake).
'cow'+'slip'='COWSLIP'
(Other definitions for cowslip that I've seen before include "Yellow flower for unsteady bovine?" , "bloomer" , "Plant with yellow flower" , "Yellow spring flower" , "Flowering plant" .)