The sergeant major is interrupted by father's involuntary movement (5)
I believe the answer is:
spasm
'involuntary movement' is the definition.
(I've seen this in another clue)
'the sergeant major is interrupted by father's' is the wordplay.
'the sergeant major' becomes 'SM' (abbreviation for sergeant major).
'is interrupted by' is an insertion indicator.
'father's' becomes 'pa's' (pa can mean dad or father).
'sm' going around 'pas' is 'SPASM'.
(Other definitions for spasm that I've seen before include "Sudden muscular contraction" , "sudden attack" , "Involuntary muscular contraction" , "sudden movement" , "Muscle twitch" .)