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' indicates taking half.
'at' is a charade indicator (letters next to each other).
'the poles' becomes 'ns' (North and South poles).
'winter' with half the letters taken 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" .)