Duty of old star from the east (4)
I believe the answer is:
onus
'duty' is the definition.
(I've seen this in another clue)
'old star from the east' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'star' becomes 'sun' (the Sun is a star).
'from the east' is a reversal indicator.
'sun' reversed gives 'nus'.
'o'+'nus'='ONUS'
'of' acts as a link.
(Other definitions for onus that I've seen before include "Capulet opponent (R&J)" , "Encumbrance" , "albatross?" , "'Burden, of proof (4)'" , "'Burden, load (4)'" .)