Old star making comeback is a liability (4)
I believe the answer is:
onus
'liability' is the definition.
(thesaurus)
'old star making comeback' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'star' becomes 'sun' (Sun is an example).
'making comeback' says the letters should be written in reverse.
'sun' backwards is 'nus'.
'o'+'nus'='ONUS'
'is a' is the link.
(Other definitions for onus that I've seen before include "Capulet opponent (R&J)" , "Obligation - charge" , "Weight of responsibility" , "Duty or responsibility" , "'Responsibility, task or burden (4)'" .)