Wear down English staff — key (5)
I believe the answer is:
erode
'wear down' is the definition.
(to erode is to wear down or decay)
'english staff key' is the wordplay.
'english' becomes 'E' (abbreviation as in OED).
'staff' becomes 'rod' (both can mean a stick or cane).
'key' becomes 'e' (a musical key).
'e'+'rod'+'e'='ERODE'
(Other definitions for erode that I've seen before include "Wear away with weather" , "go to pieces!" , "Gradually destroy" , "Eat away" , "Abrade" .)