Broad area in Cornish region with fluctuating heat (6)
I believe the answer is:
swathe
'broad area' is the definition.
(I've seen this before)
'cornish region with fluctuating heat' is the wordplay.
'cornish region' becomes 'SW' (South West of England).
'with' means one lot of letters go next to another.
'fluctuating' indicates an anagram.
'heat' is an anagram of 'athe'.
'sw'+'athe'='SWATHE'
'in' acts as a link.
(Other definitions for swathe that I've seen before include "Wrap; a cut strip" , "Wrap; bandage" , "Row or path, of grass perhaps, when it is mown" , "considerable cut" , "Bandage completely - band of mown grass" .)