Convince possibly Red Sea up (8)
I believe the answer is:
persuade
'convince' is the definition.
(I've seen this before)
'possibly red sea up' is the wordplay.
'possibly' indicates an anagram.
'red'+'sea'+'up'='redseaup'
'redseaup' is an anagram of 'PERSUADE'.
(Other definitions for persuade that I've seen before include "Bring round" , "Coax, prevail upon" , "'Coax, inveigle (8)'" , "Cause (someone) to believe" , "Convince or induce, maybe coax" .)