So concerned with partner (6)
I believe the answer is:
really
'so' is the definition.
(thesaurus)
'concerned with partner' is the wordplay.
'concerned with' becomes 're'.
'partner' becomes 'ally' (I've seen this in other clues**).
're'+'ally'='REALLY'
(Other definitions for really that I've seen before include "Well I never!" , "In truth - join forces again" , "Go on" , "Actually, honest to God" , "Actually, in fact" .)