Rebellious politician breaks pledge for high post (7)
I believe the answer is:
topmast
'high post' is the definition.
(I've seen this before)
'rebellious politician breaks pledge' is the wordplay.
'rebellious' is a reversal indicator.
'politician' becomes 'mp' (Member of Parliament).
'breaks' indicates putting letters inside.
'pledge' becomes 'toast' (synonyms).
'mp' reversed gives 'pm'.
'pm' going into 'toast' is 'TOPMAST'.
'for' acts as a link.
(Other definitions for topmast that I've seen before include "post on board" , "Most apt (anag) - feature of a square-rigged ship" , "Sail-supporting section of a ship" , "High part of sailing ship" , "Part of square-rigged ship" .)