Girl in wicked love head over heels? (5)
I believe the answer is:
olive
'girl' is the definition.
'wicked love head over heels?' is the wordplay.
'wicked' becomes 'evil' (synonyms).
'love' becomes 'o' (love means zero in tennis).
'head over heels?' shows that the letters should be reversed in order (in a down clue, letters flip upside down).
'evil'+'o'='evilo'
'evilo' in reverse letter order is 'OLIVE'.
'in' is the link.
(Other definitions for olive that I've seen before include "It's branch is a symbol of peace" , "Cocktail fruit" , "Small green tree-fruit, nice in martini" , "Small green Mediterranean tree-fruit (5)" , "woman" .)