Confined though unlikely to put on weight (5)
I believe the answer is:
local
'confined' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for local that I've seen before include "Belonging to a particular place" , "Regional; a pub" , "where the neighbours drink?" , "hearing" , "nearest boozer!" .)