Is livid to watch The Saphead (7)
I believe the answer is:
seethes
'is livid' is the definition.
(to seethe is to be angry)
'watch the saphead' is the wordplay.
'watch' becomes 'see' (synonyms).
'saphead' becomes 's' (head letter of 'sap').
'see'+'the'+'s'='SEETHES'
'to' is the link.
(Other definitions for seethes that I've seen before include "is boiling" , "furious" , "is angry" , "gets agitated" , "'Boils, with anger perhaps (7)'" .)