Infatuated with best man, harbouring love (8)
I believe the answer is:
besotted
'infatuated with' is the definition.
(I've seen this before)
'best man harbouring love' is the wordplay.
'man' becomes 'ted' (Ted is a man's name**).
'harbouring' means one lot of letters goes inside another.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'best'+'ted'='bestted'
'bestted' going around 'o' is 'BESOTTED'.
(Other definitions for besotted that I've seen before include "Doting" , "Showing foolish fondness" , "Bested to become infatuated" , "Smitten" , "Head over heels in love" .)