Realize, after losing six balls: it's a record (4)
I believe the answer is:
disc
'it's a record' is the definition.
(eg a CD, compact disc)
'realize after losing six balls' is the wordplay.
'realize' becomes 'discover'.
'after losing' indicates named letters should be taken away.
'six balls' becomes 'over' (cricket).
'discover' with 'over' removed is 'DISC'.
(Other definitions for disc that I've seen before include "Capital of India" , "Thin flat circular object" , "CD perhaps" , "Flat, circular plate - record" , "'Round, flat shape (4)'" .)