Murphy maybe, who rides the underground? (5)
I believe the answer is:
tuber
'murphy maybe who rides' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'the underground?' is the wordplay.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'underground?' becomes 'uber' (I am not sure about this - if you are sure you should believe this answer much more).
't'+'uber'='TUBER'
Can you help me to learn more?
(Other definitions for tuber that I've seen before include "Brute (anag)" , "Thickened fleshy root" , "A potato, for instance" , "Potato, for example" , "Brute or a potato plant" .)