Sergeant Major joining health resort to get fit (5)
I believe the answer is:
spasm
'fit' is the definition.
(I've seen this before)
'sergeant major joining health resort' is the wordplay.
'sergeant major' becomes 'sm' (abbreviation for sergeant major).
'joining' is a charade indicator (letters next to each other).
'health resort' becomes 'spa' (I've seen this before).
'sm' put after 'spa' is 'SPASM'.
'to get' is the link.
(Other definitions for spasm that I've seen before include "sudden attack" , "sudden movement" , "Fit" , "Sudden muscular contraction" , "Burst" .)