Truly concerning a friend (6)
I believe the answer is:
really
'truly' is the definition.
('really' can be a synonym of 'truly')
'concerning a friend' is the wordplay.
'concerning' becomes 're'.
'a friend' becomes 'ally' ('ally' can be a synonym of 'friend'**).
're'+'ally'='REALLY'
(Other definitions for really that I've seen before include "are you sure?" , "without a doubt" , "In genuine way" , "as a matter of fact" , "Well I never!" .)