Resort to sergeant-major to get fit (5)
I believe the answer is:
spasm
'fit' is the definition.
(I've seen this in another clue)
'resort to sergeant-major' is the wordplay.
'resort' becomes 'spa' (spa is a kind of resort).
'to sergeant-major' becomes 'sm' (military abbreviation).
'spa'+'sm'='SPASM'
'to get' acts as a link.
(Other definitions for spasm that I've seen before include "sudden attack" , "Muscular convulsion" , "cramp" , "Involuntary muscular contraction" , "Convulsive action" .)