Suggestive behaviour, maybe, with French kept in proportion (6,8)
I believe the answer is:
vulgar fraction
'proportion' is the definition.
'suggestive behaviour maybe with french kept in' is the wordplay.
'suggestive' becomes 'vulgar'.
'behaviour maybe' becomes 'action'.
'with' means one lot of letters go next to another.
'french' becomes 'fr'.
'kept in' means one lot of letters goes inside another.
'vulgar'+'action'='vulgaraction'
'vulgaraction' placed around 'fr' is 'VULGAR FRACTION'.
(Other definitions for vulgar fraction that I've seen before include "in which division is evident" , "Mathematical expression" , "in bar between five and six, for example" .)