He has only a penny to contribute, at the end of the day (5)
I believe the answer is:
paddy
'he has only a' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'penny to contribute at the end of the day' is the wordplay.
'penny' becomes 'p' (short for pence).
'to contribute' becomes 'add' ('add' can be a synonym of 'contribute').
'at the end of' says to take the final letters.
The last letter of 'day' is 'y'.
'p'+'add'+'y'='PADDY'
Can you help me to learn more?
(Other definitions for paddy that I've seen before include "Rice-growing field" , "Rice in the husk" , "One regularly flooded" , "show of anger" , "Wobbly" .)