Endlessly shout after sailor enters party (5)
I believe the answer is:
cabal
'party' is the definition.
(I've seen this before)
'endlessly shout after sailor enters' is the wordplay.
'endlessly' means to remove the last letter.
'shout' becomes 'call' (synonyms).
'after sailor' becomes 'ab' (abbreviation for able seaman. I am not sure about the 'after' bit.).
'enters' means one lot of letters goes inside another.
'call' with its final letter taken off is 'cal'.
'cal' enclosing 'ab' is 'CABAL'.
(Other definitions for cabal that I've seen before include "Camp" , "Secret political clique or faction" , "Intriguing group" , "Clique of plotters" , "Secret plot; clique" .)