Isn’t off beat in it for a moment (7)
I believe the answer is:
instant
'a moment' is the definition.
('instant' can be a synonym of 'moment')
'isn't off beat in it' is the wordplay.
'off' is an anagram indicator.
'beat' becomes 'tan' (I've seen this before).
'in it' indicates putting letters inside.
'isnt' anagrammed gives 'inst'.
'inst' enclosing 'tan' is 'INSTANT'.
'for' acts as a link.
(Other definitions for instant that I've seen before include "Not detailed" , "Occurring with no delay" , "Brief moment" , "Moment - this month" , "Tick" .)