Tom's apt to upset supporter (7)
I believe the answer is:
topmast
'supporter' is the definition.
'tom's apt to upset' is the wordplay.
'to upset' is an anagram indicator.
'toms'+'apt'='tomsapt'
'tomsapt' is an anagram of 'TOPMAST'.
(Other definitions for topmast that I've seen before include "Most apt (anag) - feature of a square-rigged ship" , "Sail-supporting section of a ship" , "High part of sailing ship" , "post on board" , "high post" .)