Restriction also includes a bachelor (5)
I believe the answer is:
taboo
'restriction' is the definition.
(I've seen this in another clue)
'also includes a bachelor' is the wordplay.
'also' becomes 'too' (synonyms).
'includes' is an insertion indicator.
'bachelor' becomes 'B' (abbreviation e.g. BA=Bachelor of Arts).
'a'+'b'='ab'
'too' enclosing 'ab' is 'TABOO'.
(Other definitions for taboo that I've seen before include "No-no" , "Prohibited as unclean or holy" , "Prohibited by social custom" , "Subject to prohibition" , "Ritual prohibition" .)