Emperor, say, being entertained by rising star (5)
I believe the answer is:
negus
'emperor' is the definition.
(I've seen this before)
'say being entertained by rising star' is the wordplay.
'say' becomes 'eg' (both can mean 'for example').
'being entertained by' indicates putting letters inside (inserted letters are hosted or entertained).
'rising' is a reversal indicator (in a down clue, letters go up).
'star' becomes 'sun' (the Sun is a star).
'sun' backwards is 'nus'.
'eg' going inside 'nus' is 'NEGUS'.
(Other definitions for negus that I've seen before include "Title of Emperor of Ethiopia" , "Hot wine" , "Hot fortified drink" , "Ruler in Africa" , "Warm drink" .)