Heather’s endlessly head over heels in love (3)
I believe the answer is:
nil
'love' is the definition.
(love and nil mean zero in cricket and football)
'heather's endlessly head over heels' is the wordplay.
'heather' becomes 'ling' (synonyms).
'endlessly' means to remove the last letter.
'head over heels' says the letters should be written backwards (in a down clue, letters flip upside down).
'ling' with its final letter removed is 'lin'.
'lin' in reverse letter order is 'NIL'.
'in' is the link.
(Other definitions for nil that I've seen before include "Non-existent" , "duck" , "Nothing, zilch" , "Zero, no score" , "Don't despair - . . . desperandum" .)