Seed: just what’s needed for bird (5)
I believe the answer is:
pipit
'for bird' is the definition.
(pipit is a kind of bird)
'seed just what's needed' is the wordplay.
'seed' becomes 'pip' (as in a fruit pip).
'just what's needed' becomes 'it' (as in 'that's it!').
'pip'+'it'='PIPIT'
(Other definitions for pipit that I've seen before include "Songbird of open country" , "Lark-like bird" , "Songbird of rock or meadow" , "Bird of meadow, tree, or rock" , "Lark-like songbird" .)