Vagrant, mouth full of marshmallows, primarily (5)
I believe the answer is:
tramp
'vagrant' is the definition.
(thesaurus)
'mouth full of marshmallows primarily' is the wordplay.
'mouth' becomes 'trap' (informal term for mouth).
'full of' indicates putting letters inside.
'primarily' suggests taking the first letters.
The first letter of 'marshmallows' is 'm'.
'trap' placed around 'm' is 'TRAMP'.
(Other definitions for tramp that I've seen before include "Vagrant - cargo ship" , "Footslog" , "Vagrant, drifter" , "Bum!" , "Walk heavily like hobo" .)