If very soft-hearted, he'd make a tasty contribution (4)
I believe the answer is:
peer
'if very soft-hearted he'd' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'make a tasty contribution' is the wordplay.
'make' becomes 'pee' (synonyms).
'a tasty contribution' becomes 'r' (I can't justify this - if you can you should give a lot more credence to this answer).
'pee'+'r'='PEER'
Can you help me to learn more?
(Other definitions for peer that I've seen before include "Look closely - nobleman" , "Equal; nobleman" , "one visiting ladies, say?" , "Peep; nobleman" , "Look hard at the nobleman" .)