Quite concerned with friend (6)
I believe the answer is:
really
'quite' is the definition.
(synonyms)
'concerned with friend' is the wordplay.
'concerned with' becomes 're' (re can mean regarding or about).
'friend' becomes 'ally' (an ally is a friend or supporter).
're'+'ally'='REALLY'
(Other definitions for really that I've seen before include "Well I never!" , "Honestly, it's a fact" , "Actually, honest to God" , "Do you mean it?" , "as a matter of fact" .)