He made large mushrooms ripe with one hemp fusion? (11)
I believe the answer is:
oppenheimer
'he made large mushrooms ripe with one hemp fusion?' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he made large mushrooms ripe with one' is the wordplay.
I cannot quite understand how this works, but
'he' could be 'pen' and 'pen' is found in the answer.
'one' could be 'me' (a setter might use 'one' to humourously mean 'me') and 'me' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for oppenheimer that I've seen before include "Atomic expert" , "Nuclear physicist" .)