Average returned, left in sympathetic connection (7)
I believe the answer is:
'sympathetic connection' is the definition.
(I've seen this in another clue)
'average returned left' is the wordplay.
'average' becomes 'par' (average score in golf).
'returned' shows that the letters should be reversed in order.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'par' backwards is 'rap'.
'in' is the link.
(Other definitions for rapport that I've seen before include "Close relationship with a common understanding" , "Mutual harmony and affinity" , "Instinctive understanding between people" , "Empathy" , "Relation, connection" .)