Responsibility of star making a comeback after Oscar (4)
I believe the answer is:
onus
'responsibility' is the definition.
(onus is a duty or responsibility)
'star making a comeback after oscar' is the wordplay.
'star' becomes 'sun' (Sun is an example).
'making a comeback' shows that the letters should be reversed in order.
'after' means one lot of letters go next to another.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'sun' back-to-front is 'nus'.
'nus' put after 'o' is 'ONUS'.
'of' acts as a link.
(Other definitions for onus that I've seen before include "'Responsibility, task or burden (4)'" , "Responsibilty" , "Encumbrance" , "Weight of responsibility" , "Capulet opponent (R&J)" .)