A gym Ali used for muscular pain (7)
I believe the answer is:
myalgia
'muscular pain' is the definition.
(I've seen this before)
'a gym ali used' is the wordplay.
'used' indicates an anagram.
'a'+'gym'+'ali'='agymali'
'agymali' with letters rearranged gives 'MYALGIA'.
'for' acts as a link.
(Other definitions for myalgia that I've seen before include "sort of pain!" , "painful muscle condition" , "Muscular pain" , "Muscle pain" .)