Wheat is a smile on the outside (5)
I believe the answer is:
grain
'wheat' is the definition.
(thesaurus)
'a smile on the outside' is the wordplay.
'smile' becomes 'grin' (synonyms).
'on the outside' means one lot of letters goes inside another.
'a' going inside 'grin' is 'GRAIN'.
'is' acts as a link.
(Other definitions for grain that I've seen before include "Single piece of sand etc" , "Silo contents" , "Particle of e.g. sand" , "Tiny amount" , "Small hard particle of anything" .)