Prohibition also involves sailor (5)
I believe the answer is:
taboo
'prohibition' is the definition.
(thesaurus)
'also involves sailor' is the wordplay.
'also' becomes 'too' ('too' can be a synonym of 'also').
'involves' means one lot of letters goes inside another.
'sailor' becomes 'ab' (abbreviation for able seaman).
'too' going around 'ab' is 'TABOO'.
(Other definitions for taboo that I've seen before include "Not allowed, forbidden, proscribed" , "By religion or custom, not to be done or touched" , "ruled out" , "Social prohibition" , "unacceptable" .)