Rhythm method primarily interrupts coupling (4)
I believe the answer is:
time
'rhythm' is the definition.
(time is a kind of rhythm)
'method primarily interrupts coupling' is the wordplay.
'primarily' says to take the initial letters.
'interrupts' indicates putting letters inside.
'coupling' becomes 'tie' (tie is a type of coupling or connection).
The initial letter of 'method' is 'm'.
'm' placed within 'tie' is 'TIME'.
(Other definitions for time that I've seen before include "'Every ... we say goodbye, I cry a little' /Cole Porter" , "Procrastination is the thief of ." , "Spell" , "Finite duration" , "Its detonation is delayed" .)