Make firm but possibly get thin (7)
I believe the answer is:
tighten
'make firm' is the definition.
(tighten can mean to make firm or hard)
'possibly get thin' is the wordplay.
'possibly' indicates anagramming the letters.
'get'+'thin'='getthin'
'getthin' is an anagram of 'TIGHTEN'.
'but' acts as a link.
(Other definitions for tighten that I've seen before include "Make harder to undo" , "contract" , "Apply increased tension or security" , "To improve focus" , "Constrict" .)