Rolling barrel contains Mike Walker (7)
I believe the answer is:
rambler
'walker' is the definition.
(I know that rambler is a type of walker)
'rolling barrel contains mike' is the wordplay.
'rolling' indicates an anagram.
'contains' indicates putting letters inside.
'mike' becomes 'M' (phonetic alphabet: alpha, bravo, charlie etc.).
'barrel' with letters rearranged gives 'rabler'.
'rabler' enclosing 'm' is 'RAMBLER'.
(Other definitions for rambler that I've seen before include "Straggling rose" , "Person who takes long walks in the country" , "Rose - that goes for country walks?" , "One who goes on country walks" , "country activist" .)