Female relative invading pitch is showing off (9)
I believe the answer is:
flaunting
'is showing off' is the definition.
(I've seen this before)
'female relative invading pitch' is the wordplay.
'female relative' becomes 'aunt' (I've seen this before).
'invading' indicates putting letters inside.
'pitch' becomes 'fling'.
'aunt' placed within 'fling' is 'FLAUNTING'.
(Other definitions for flaunting that I've seen before include "Showing off" , "Ostentatiously displaying" .)