Being emperor was somewhat onerous (4)
I believe the answer is:
nero
'being emperor' is the definition.
(Roman emperor)
'somewhat onerous' is the wordplay.
'somewhat' indicates the answer is hidden within the clue.
'NERO' is hidden in the letters of 'onerous'.
'was' acts as a link.
(Other definitions for nero that I've seen before include "Imperial Roman fiddler" , "Matricidal emperor" , "Emperor (who fiddled while Rome burned?) d. AD68" , "Ruler" , "Roman Emperor notorious for vice and luxury" .)