Overweight? Get back into shape! (5)
I believe the answer is:
plump
'overweight?' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for plump that I've seen before include "Portly" , "Opt - round" , "Rather fat" , "Chubby, fat" , "Chubby, round" .)