Unusual formation of geese in winter season (5)
I believe the answer is:
novel
'unusual' is the definition.
(I've seen this in another clue)
'formation of geese in winter season' is the wordplay.
'formation of geese' becomes 'V' (geese might fly in a V formation).
'in' indicates putting letters inside.
'winter season' becomes 'Noel' (Noel can mean Christmas).
'v' placed inside 'noel' is 'NOVEL'.
(Other definitions for novel that I've seen before include "A work of fiction of new kind" , "New; original" , "New and strange" , "New sort of fiction?" , "Unusual; book" .)