Iron Duke had a load on his plate (3)
I believe the answer is:
fed
'a load on his plate' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'iron duke' is the wordplay.
'iron' becomes 'fe' (chemical symbol for iron).
'duke' becomes 'd' (abbreviation).
'fe'+'d'='FED'
'had' acts as a link.
Can you help me to learn more?
(Other definitions for fed that I've seen before include "Given food to eat" , "Banking authority" , "Provided courses" , "US agent" , "supplied with power?" .)