Jersey's post collection (7)
I believe the answer is:
maillot
'jersey's' is the definition.
(I've seen this before)
'post collection' is the wordplay.
'post' becomes 'mail' (synonyms).
'collection' becomes 'lot' (lot is a kind of collection).
'mail'+'lot'='MAILLOT'
(Other definitions for maillot that I've seen before include "Top" , "Swimsuit" , "Jersey" , "Tights worn for ballet" .)