Refuse to get thin (5)
I believe the answer is:
waste
This clue is a triple definition!
'refuse' is the first definition.
(I know that refuse can be written as waste)
'to get' is the second definition.
(thesaurus)
'thin' is the third definition.
(thesaurus)
(Other definitions for waste that I've seen before include "''.... not, want not''" , "Fritter away" , "Blue" , "Spend to no purpose" , "Not use effectively" .)