Well, I messed up with gala (muscle pain) (7)
I believe the answer is:
myalgia
'muscle pain' is the definition.
(I've seen this in another clue)
'well i messed up with gala' is the wordplay.
'well' becomes 'my' (both are expressions of surprise).
'messed up with' indicates an anagram.
'i' anagrammed with 'gala' gives 'algia'.
'my'+'algia'='MYALGIA'
(Other definitions for myalgia that I've seen before include "painful muscle condition" , "Muscle pain" , "sort of pain!" , "Muscular pain" .)