Genuinely concerned with colleague (6)
I believe the answer is:
really
'genuinely' is the definition.
(I know that genuinely can be written as really)
'concerned with colleague' is the wordplay.
'concerned with' becomes 're' (re can mean regarding, concerning).
'colleague' becomes 'ally' (ally can mean a colleague or associate).
're'+'ally'='REALLY'
(Other definitions for really that I've seen before include "Do you mean it?" , "Indeed" , "In truth - one may be surprised to say it" , "Actually, truly" , "Truly, in fact" .)