Flower an animal found by mistake (7)
I believe the answer is:
cowslip
'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' is a charade indicator (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 "Wild yellow primula" , "Wild spring flower" , "Pretty little wild flower for unsteady bovine" , "Yellow wild flower" , "bloomer" .)