Murphy, maybe? (5)
I believe the answer is:
tuber
'murphy' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for tuber that I've seen before include "Thickened fleshy root" , "A potato, for instance" , "Brute (anag)" , "Root of potato etc" , "Root eg of potato" .)