English bird's wintry, occasionally for ages (8)
I believe the answer is:
eternity
'ages' is the definition.
(thesaurus)
'english bird's wintry occasionally' is the wordplay.
'english' becomes 'e' (abbreviation).
'bird' becomes 'tern' (tern is a kind of bird**).
'occasionally' means one should take alternating letters.
The alternate letters of 'wintry' are 'ity'.
'e'+'tern'+'ity'='ETERNITY'
'for' is the link.
(Other definitions for eternity that I've seen before include "Endless duration" , "An extremely long time" , "the poet palmed in an hour?" , "That's a very long time" , "timelessness" .)