Underwear ultimately unsuitable for a northern hillside (4)
I believe the answer is:
brae
'a northern hillside' is the definition.
'underwear ultimately unsuitable' is the wordplay.
'underwear' becomes 'bra' (bra is a type of undergarment).
'ultimately' suggests the final letters.
The last letter of 'unsuitable' is 'e'.
'bra'+'e'='BRAE'
'for' acts as a link.
(Other definitions for brae that I've seen before include "Scottish hill slope" , "Bare on Scottish hillside" , "Steep bank or hillside" , "Bear up on Scottish hillside" , "Slope on hill or in glen" .)