Burden of love to rising star (4)
I believe the answer is:
onus
'burden' is the definition.
(I know that burden can be written as onus)
'love to rising star' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'to rising' says the letters should be written in reverse (letters written upwards in a down clue).
'star' becomes 'sun' (the Sun is a star).
'sun' back-to-front is 'nus'.
'o'+'nus'='ONUS'
'of' acts as a link.
(Other definitions for onus that I've seen before include "'Burden, load (4)'" , "Obligation - charge" , "albatross?" , "'Responsibility, task or burden (4)'" , "Encumbrance" .)