Very concerned with partner (6)
I believe the answer is:
really
'very' is the definition.
(I've seen this before)
'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 "Honestly, it's a fact" , "Actually, no kidding" , "Actually, in fact" , "without a doubt" , "In genuine way" .)