Give in concerning fast (6)
I believe the answer is:
relent
'give in' is the definition.
(I've seen this before)
'concerning fast' is the wordplay.
'concerning' becomes 're'.
'fast' becomes 'lent' (I've seen this before).
're'+'lent'='RELENT'
(Other definitions for relent that I've seen before include "Be merciful" , "Become less severe and more forgiving" , "Moderate" , "Abandon cruel treatment" , "'Give in, soften (6)'" .)