Sign used by vendors everywhere (7)
I believe the answer is:
endorse
'sign' is the definition.
(I know that endorse is a more specific form of the action sign)
'used by vendors everywhere' is the wordplay.
'used by' indicates the answer is hidden within the clue.
'ENDORSE' can be found hidden inside 'vendors everywhere'.
(Other definitions for endorse that I've seen before include "Champion" , "Ratify" , "Say you approve" , "Approve or underwrite" , "Sore end (anag.)" .)