Spar, a post most hollow at sea (7)
I believe the answer is:
topmast
'spar' is the definition.
(topmast is a kind of spar)
'a post most hollow at sea' is the wordplay.
'hollow' suggests removing the centre.
'at sea' is an anagram indicator.
'most' with its middle removed is 'mt'.
'a'+'post'+'mt'='apostmt'
'apostmt' with letters rearranged gives 'TOPMAST'.
(Other definitions for topmast that I've seen before include "Most apt (anag) - feature of a square-rigged ship" , "Sail up here" , "High part of sailing ship" , "Sail-supporting section of a ship" , "Part of square-rigged ship" .)