Timeworn stratagem in English county not entirely backed (8)
I believe the answer is:
overused
'timeworn' is the definition.
(timeworn means overused or cliched)
'stratagem in english county not entirely backed' is the wordplay.
'stratagem' becomes 'ruse' (a ruse is a stratagem or ploy).
'in' is an insertion indicator.
'english county' becomes 'Devon' (Devon is an English county).
'not entirely' means to remove the last letter (not all of the word).
'backed' says the letters should be written backwards.
'devon' with its final letter taken away is 'devo'.
'devo' back-to-front is 'oved'.
'ruse' inserted into 'oved' is 'OVERUSED'.
(Other definitions for overused that I've seen before include "Revues do tend to be excessively utilised" , "Employed excessively for tasks, probably worn" , "Hackneyed" .)