So loveless at heart, he was a saint! (7)
I believe the answer is:
swithin
'a saint' is the definition.
(saint swithun/swithin)
'so loveless at heart he' is the wordplay.
I cannot really see how this works, but
'heart' could be 'h' (card game abbreviation) and 'h' is located in the answer.
'he' could be 'wit' and 'wit' is present in the answer.
The remaining letters 'sin' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'was' acts as a link.
Can you help me to learn more?
(Other definitions for swithin that I've seen before include "Saint; with sin (anag,)" , "Saint associated with weather (I not U)" .)