Emperor’s son welcomed by sailor (4)
I believe the answer is:
tsar
'emperor's' is the definition.
(I've seen this before)
'son welcomed by sailor' is the wordplay.
'son' becomes 's'.
'welcomed by' means one lot of letters goes inside another (I've seen this in other clues).
'sailor' becomes 'tar' (tar is a kind of sailor).
's' inserted within 'tar' is 'TSAR'.
(Other definitions for tsar that I've seen before include "Despot" , "Title of Russian emperor" , "Ruler finally executed" , "Male monarch" , "Eg, Ivan the Terrible" .)