Give up a prohibition mafia boss (7)
I believe the answer is:
abandon
'give up' is the definition.
(synonyms)
'a prohibition mafia boss' is the wordplay.
'prohibition' becomes 'ban' (synonyms).
'mafia boss' becomes 'don' (term for a mafia leader).
'a'+'ban'+'don'='ABANDON'
(Other definitions for abandon that I've seen before include "Desert, strand" , "Stop" , "Desert or yield without restraint" , "Leave high and dry" , "Desert; intemperance" .)