What’s found inside vendor sent back (7)
I believe the answer is:
endorse
'back' is the definition.
(synonyms)
'what's found inside vendor sent' is the wordplay.
'what's found' indicates a hidden word.
'ENDORSE' is hidden within 'inside vendor sent'.
(Other definitions for endorse that I've seen before include "Give support or approval to" , "Favour" , "Approve of, sign on back as of cheque" , "Red ones (anag.)" , "Give public approval of (something)" .)