Substitute old piece in county briefly (9)
I believe the answer is:
surrogate
'substitute' is the definition.
(surrogate is a kind of substitute)
'old piece in county briefly' is the wordplay.
I cannot quite understand how this works, but
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is found in the answer.
'piece' could be 'gat' (both can mean a gun) and 'gat' is found within the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for surrogate that I've seen before include "stand-in" , "Deputising substitute" , "(A) substitute in office" , "Substitute fulfilling the role of another" , "Argues about rot with substitute or deputy" .)