He may have a second helping (5)
I believe the answer is:
boxer
'he may' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'a second helping' is the wordplay.
'a second' becomes 'b' (B can mean second eg B-side).
'helping' becomes 'oxer' (I can't justify this - if you can you should believe this answer much more).
'b'+'oxer'='BOXER'
'have' is the link.
Can you help me to learn more?
(Other definitions for boxer that I've seen before include "Tyson, perhaps - type of dog" , "Person assisted by a second" , "Dog - fighter" , "Asian rebel" , "Pugilist - dog" .)