This may be hot or cold and mountainous, we hear (6)
I believe the answer is:
chilli
'this may be hot' is the definition.
(I know this)
'cold and mountainous we hear' is the wordplay.
'cold' becomes 'c' (eg on taps).
'and' means one lot of letters go next to another.
'mountainous' becomes 'hilly' (synonyms).
'we hear' indicates a 'sounds like' (homophone) clue.
'c'+'hilly'='chilly'
'chilly' sounds like 'CHILLI'.
'or' acts as a link.
(Other definitions for chilli that I've seen before include "Hot-tasting red pepper used as relish" , "could make a hot meal" , "Give up completely" , "Hot one in dish maybe" , "'Red, green or yellow pepper (6)'" .)