Very concerning associate (6)
I believe the answer is:
really
'very' is the definition.
(I've seen this before)
'concerning associate' is the wordplay.
'concerning' becomes 're'.
'associate' becomes 'ally' (ally is a kind of associate**).
're'+'ally'='REALLY'
(Other definitions for really that I've seen before include "Actually, honest to God" , "In truth - one may be surprised to say it" , "Genuinely" , "In genuine way" , "Honestly, it's a fact" .)