Bank filling boxes for birds (8)
I believe the answer is:
sparrows
'birds' is the definition.
(sparrow is a kind of bird)
'bank filling boxes' is the wordplay.
'bank' becomes 'row' (I've seen this in other clues).
'filling' means one lot of letters goes inside another.
'boxes' becomes 'spars' (sparring is a kind of boxing).
'row' going within 'spars' is 'SPARROWS'.
'for' acts as a link.
(Other definitions for sparrows that I've seen before include "These birds warp Ross badly" , "Small brown birds" , "Small singing birds" .)