Plain Cockney tough guy from the East End (6)
I believe the answer is:
tundra
'plain' is the definition.
(I know that tundra is a type of plain)
'cockney tough guy from the east end' is the wordplay.
'cockney tough guy' becomes ''ardnut' ('hardnut' - informal term for an aggressive person - said with the 'H' dropped - a cockney accent.).
'from the east end' is a reversal indicator (from east to west).
'ardnut' written backwards gives 'TUNDRA'.
(Other definitions for tundra that I've seen before include "Area with frozen subsoi" , "Treeless Arctic region" , "Arctic region of permafrost" , "Area of permafrost" , "Artic treeless plain" .)