Young Murphy used to make others supply a dope test? Nothing in that (4,6)
I believe the answer is:
seed potato
'young murphy used to make others' is the definition.
(murphy is slang for potato)
'supply a dope test? nothing in that' is the wordplay.
'supply' indicates anagramming the letters (in a supple way).
'nothing' becomes 'O' (O resembles 0).
'in that' means one lot of letters goes inside another.
'a'+'dope'+'test'='adopetest'
'adopetest' is an anagram of 'seedptato'.
'seedptato' placed around 'o' is 'SEED POTATO'.
(Another definition for seed potato that I've seen is " Vegetable".)