The poor, it could be heartless to deny! (5)
I believe the answer is:
needy
'the poor' is the definition.
(I've seen this before)
'it could be heartless to deny' is the wordplay.
I cannot really understand how this works, but
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is found within the answer.
an anagram of 'deny' is 'nedy' which is present in the left over letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for needy that I've seen before include "Insecure; very poor" , "Disadvantaged" , "Distressed" , "Penurious" , "Wanting attention" .)