Principal boy embraces heroine at last (4)
I believe the answer is:
lead
'principal' is the definition.
('lead' can be a synonym of 'principal')
'boy embraces heroine at last' is the wordplay.
'boy' becomes 'lad' (lad is a kind of boy**).
'embraces' means one lot of letters goes inside another.
'at last' indicates one should take the final letters.
The last letter of 'heroine' is 'e'.
'lad' enclosing 'e' is 'LEAD'.
(Other definitions for lead that I've seen before include "Principal performer" , "Be ahead of - guide" , "Chemical element, symbol Pb" , "initiative" , "Guide, conduct" .)