Second animal left a frown (5)
I believe the answer is:
scowl
'a frown' is the definition.
('scowl' can be a synonym of 'frown')
'second animal left' is the wordplay.
'second' becomes 's' (common abbreviation).
'animal' becomes 'cow' (cow is a kind of animal**).
'left' becomes 'l' (common abbreviation).
's'+'cow'+'l'='SCOWL'
(Other definitions for scowl that I've seen before include "Glower; frown" , "Angry facial expression" , "Look angry" , "dirty look" , "Look threatening" .)