Avoid small birds (5)
I believe the answer is:
ducks
'birds' is the definition.
(duck is a kind of bird)
'avoid small' is the wordplay.
'avoid' becomes 'duck' (duck can mean to avoid).
'small' becomes 's' (abbreviation - e.g. clothes size).
'duck'+'s'='DUCKS'
(Other definitions for ducks that I've seen before include "Swimming birds with scores of nothing?" , "Crouches" , "My pet" , "dodges" , "Lowers head suddenly" .)