Plump bird associated with Christmas competition (5,5)
I believe the answer is:
round robin
'competition' is the definition.
(round robin is a kind of competition)
'plump bird associated with christmas' is the wordplay.
'plump' becomes 'round' (I've seen this before).
'bird associated with christmas' becomes 'robin'.
'round'+'robin'='ROUND ROBIN'
(Other definitions for round robin that I've seen before include "Multi-signature letter" , "Circular letter, petition" , "Tournament in which every contestant plays against every other" , "Statement signed" , "Where mourning birds gathered" .)