Part of a long poem one is unable to love? (5)
I believe the answer is:
canto
'part of a long poem' is the definition.
(I've seen this in another clue)
'one is unable to love?' is the wordplay.
'one is unable to' becomes 'cant' (can't).
'love?' becomes 'o' (love means zero in tennis).
'cant'+'o'='CANTO'
(Other definitions for canto that I've seen before include "piece of poetry" , "Musical part" , "Division of e.g. Dante's Inferno" , "Portion of poem" , "Highest part in a piece of choral music -- act on (anag)" .)