Snug, wings of goldfinch tucked into bird (5)
I believe the answer is:
tight
'snug' is the definition.
(similar in meaning)
'wings of goldfinch tucked into bird' is the wordplay.
'wings of' suggests removing the centre (letters on the two wings removed).
'tucked into' is an insertion indicator.
'bird' becomes 'tit' (tit is a kind of bird).
'goldfinch' with its centre removed is 'gh'.
'gh' put into 'tit' is 'TIGHT'.
(Other definitions for tight that I've seen before include "happy" , "Drunk - a near thing!" , "Steaming" , "Hard to undo or open" , "Well oiled" .)