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