Staff of American Express (4)
I believe the answer is:
cane
'staff' is the definition.
(I know this)
'of american express' is the wordplay.
'of' indicates a hidden word.
'CANE' can be found hidden inside 'american express'.
(Other definitions for cane that I've seen before include "Stick; thrash" , "Slender stick" , "Sugar plant" , "not beet" , "Strong stem for to allow a plant to climb" .)