Birds half the winter at the poles (5)
I believe the answer is:
terns
'birds' is the definition.
(tern is a kind of bird)
'half the winter at the poles' is the wordplay.
'half the' indicates taking half.
'at' says to put letters next to each other.
'the poles' becomes 'ns' (North and South poles).
'winter' cut in half is 'ter'.
'ter'+'ns'='TERNS'
(Other definitions for terns that I've seen before include "Rents out these birds" , "Sea birds with long forked tails" , "Seabirds" , "Sea bird with long forked tail" , "Aquatic birds" .)