Sign, send or seal contract (7)
I believe the answer is:
endorse
'sign' is the definition.
(endorsing is a kind of signing)
'send or seal contract' is the wordplay.
'contract' indicates a hidden word (I've seen 'contracted' mean this).
'ENDORSE' can be found hidden inside 'send or seal'.
(Other definitions for endorse that I've seen before include "Sanction -- red nose (anag)" , "Rubber-stamp" , "Add one's authority to" , "Give public approval of (something)" , "Corroborate" .)