Caprice married in one beat (7)
I believe the answer is:
impulse
'caprice' is the definition.
('impulse' can be a synonym of 'caprice')
'married in one beat' is the wordplay.
'married' becomes 'm'.
'in' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'beat' becomes 'pulse' ('pulse' can be a synonym of 'beat'**).
'i'+'pulse'='ipulse'
'm' put inside 'ipulse' is 'IMPULSE'.
(Other definitions for impulse that I've seen before include "Sudden or instinctive desire" , "Strong urge to act" , "I may cause desire" , "Drive" , "Miles up (anag)" .)