Unrestricted hard nuts head up Arctic region (6)
I believe the answer is:
tundra
'arctic region' is the definition.
(I've seen this before)
'unrestricted hard nuts head up' is the wordplay.
'unrestricted hard nuts' becomes 'ard' (I am not sure about this - if you are sure you should believe this answer much more).
'head' becomes 'nut' (informal term for somebody's head).
'up' is a reversal indicator (in down clue: letters go upwards).
'ard'+'nut'='ardnut'
'ardnut' written backwards gives 'TUNDRA'.
Can you help me to learn more?
(Other definitions for tundra that I've seen before include "Cold northern region" , "wasteland" , "Sub-polar treeless zone" , "Vast level Arctic region with frozen subsoil" , "Vast treeless plane" .)