Dramatist keeps old dog (7)
I believe the answer is:
pointer
'dog' is the definition.
(pointer is a kind of dog)
'dramatist keeps old' is the wordplay.
'dramatist' becomes 'pinter' (Harold Pinter).
'keeps' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'pinter' going around 'o' is 'POINTER'.
(Other definitions for pointer that I've seen before include "Dog < or horse or" , "Gun dog" , "stick used for instruction?" , "helpful tip!" , "Tip; dog" .)