Small animal with large frown (5)
I believe the answer is:
scowl
'frown' is the definition.
(scowling is a kind of frowning)
'small animal with large' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'animal' becomes 'cow' (cow is a kind of animal).
'with' means one lot of letters go next to another.
'large' becomes 'l'.
's'+'cow'+'l'='SCOWL'
(Other definitions for scowl that I've seen before include "Angrily frown" , "Gloomy or threatening expression" , "Lower" , "Angry facial expression" , "Glower; frown" .)