Plump for a sandwich (5)
I believe the answer is:
round
I believe this is a double definition.
'plump' is the first definition.
(plump can mean round or full)
'sandwich' is the second definition.
(as in a round of sandwiches)
'for a' acts as a link.
(Other definitions for round that I've seen before include "A partsong in which voices follow each other" , "unsuitable for square pegs" , "Number of drinks bought together" , "The canon" , "Game of golf" .)