Streak of love lifted sweetheart (4)
I believe the answer is:
line
'streak' is the definition.
(thesaurus)
'love lifted sweetheart' is the wordplay.
'love' becomes 'nil' (love and nil mean zero in cricket and football).
'lifted' shows that the letters should be reversed in order (going up in down clue).
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'nil' backwards is 'lin'.
'lin'+'e'='LINE'
'of' acts as a link.
(Other definitions for line that I've seen before include "classified information" , "An American queue" , "Fill" , "Formation of one beside another" , "Policy - file" .)