Tough Henry going over freezing plain (4,3)
I believe the answer is:
hard nut
'tough' is the definition.
(hard nut can mean a tough person)
'henry going over freezing plain' is the wordplay.
'henry' becomes 'H' (symbol for the unit of electrical inductance).
'going over' says the letters should be written in reverse.
'freezing plain' becomes 'tundra' (type of geographical region).
'tundra' written backwards gives 'ardnut'.
'h'+'ardnut'='HARD NUT'
(Other definitions for hard nut that I've seen before include "A violent criminal" , "Tough person" , "It takes some cracking" .)