Smile, entertaining adult suggestion (5)
I believe the answer is:
grain
'suggestion' is the definition.
(thesaurus)
'smile entertaining adult' is the wordplay.
'smile' becomes 'grin' (synonyms).
'entertaining' means one lot of letters goes inside another.
'adult' becomes 'a' (abbreviation).
'grin' placed around 'a' is 'GRAIN'.
(Other definitions for grain that I've seen before include "Particle of e.g. sand" , "Small piece of sand" , "basis for whisky" , "little salty bit" , "Character" .)