Songbirds steal in before onset of spring (6)
I believe the answer is:
robins
'songbirds' is the definition.
(I have seen 'Small songbirds ' mean 'robins' so perhaps 'songbirds' could also mean 'robins')
'steal in before onset of spring' is the wordplay.
'steal' becomes 'rob' (robbing is a kind of stealing).
'before' is a charade indicator (letters next to each other).
'onset of' indicates taking the first letters.
The initial letter of 'spring' is 's'.
'rob'+'in'+'s'='ROBINS'
(Other definitions for robins that I've seen before include "Small thrushes" , "Birds with red breasts" , "Friendly winter birds" , "Small songbirds" , "Birds with reddish breasts" .)