Lady’s love for principal character (4)
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I believe the answer is:
hero
'principal character' is the definition.
(I've seen this before)
'lady's love' is the wordplay.
'lady' becomes 'her' (her refers to a woman).
'love' becomes 'o' (love means zero in tennis).
'her'+'o'='HERO'
'for' is the link.
(Other definitions for hero that I've seen before include "Courageous man" , "Rochester maybe required?" , "Principal male figure in a story" , "Lamp-lighting lover" , "Male protagonist" .)