Leave a large group of old boozers about to invade part of pitch (4,4)
I believe the answer is:
goal area
'part of pitch' is the definition.
(part of a football pitch)
'leave a large group of old boozers about to invade' is the wordplay.
I cannot quite understand how this works, but
'leave' could be 'go' (to go is to depart or leave) and 'go' is found in the answer.
'a' could be 'area' (maths abbreviation) and 'area' is located in the answer.
'large' could be 'l' (eg in clothes sizes) and 'l' is located in the answer.
'about' could be 'a' (abbreviation in Chambers) and 'a' is found within the answer.
No letters remain.
This explanation may well be incorrect...
(Another definition for goal area that I've seen is " Six-yard box".)