Disallow offer by pro (6)
I believe the answer is:
forbid
'disallow' is the definition.
(I know that disallow can be written as forbid)
'offer by pro' is the wordplay.
'offer' becomes 'bid' (synonyms).
'by' is a charade indicator (letters next to each other).
'pro' becomes 'for' (I've seen this in another clue).
'bid' put after 'for' is 'FORBID'.
(Other definitions for forbid that I've seen before include "bar" , "Prohibit, veto" , "Ban" , "Deny, prohibit" , "Interdict" .)