Jersey put in the post with sale item (7)
I believe the answer is:
maillot
'jersey' is the definition.
(I've seen this before)
'post with sale item' is the wordplay.
'post' becomes 'mail' (synonyms).
'with' means one lot of letters go next to another.
'sale item' becomes 'lot' (item for sale at auction).
'mail'+'lot'='MAILLOT'
'put in the' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for maillot that I've seen before include "Swimsuit" , "Top" , "Jersey" , "Tights worn for ballet" .)