Understanding to knock on the left (7)
I believe the answer is:
rapport
'understanding' is the definition.
(I've seen this before)
'knock on the left' is the wordplay.
'knock' becomes 'rap' ('rap' can be a synonym of 'knock').
'on' means one lot of letters go next to another.
'the left' becomes 'port' (left side of a ship - opposite of starboard).
'rap'+'port'='RAPPORT'
'to' is the link.
(Other definitions for rapport that I've seen before include "communion" , "Mutual harmony and affinity" , "Sympathetic understanding" , "Bond of sympathy" , "Relation, connection" .)