Set sail occasionally with popular Soviet leader (6)
I believe the answer is:
stalin
'soviet leader' is the definition.
(I've seen this before)
'set sail occasionally with popular' is the wordplay.
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
'with' says to put letters next to each other.
'popular' becomes 'in' ('in' can mean trendy).
'set'+'sail'='setsail'
The alternate letters of 'setsail' are 'stal'.
'stal'+'in'='STALIN'
(Other definitions for stalin that I've seen before include "Bolshevik" , "Instal for Russian dictator" , "Communist dictator" , "Tyrant - last in (anag)" , "Soviet dictator, d. 1953" .)