Expert covering return of last month's grant (6)
I believe the answer is:
accede
'grant' is the definition.
(thesaurus)
'expert covering return of last month's' is the wordplay.
'expert' becomes 'ace' (ace is a kind of expert).
'covering' is an insertion indicator.
'return of' shows that the letters should be reversed in order.
'last month' becomes 'dec'.
'dec' backwards is 'ced'.
'ace' enclosing 'ced' is 'ACCEDE'.
(Other definitions for accede that I've seen before include "Allow, give consent" , "Agree (to) - come into office" , "Assume" , "Agree or enter into office" , "Say yes to request" .)