Seed diet oddly not for songbird (5)
I believe the answer is:
pipit
'songbird' is the definition.
(I've seen this before)
'seed diet oddly not' is the wordplay.
'seed' becomes 'pip' (I've seen this before).
'oddly not' indicates alternate letters (not the letters in odd-numbered positions).
The alternate letters of 'diet' are 'it'.
'pip'+'it'='PIPIT'
'for' is the link.
(Other definitions for pipit that I've seen before include "Small bird of rock or meadow" , "Titlark" , "Lark-like songbird" , "Songbird with streaky brown plumage" , "Bird related to wagtail" .)