Water heater, nasty grey, seen around Home Counties (6)
I believe the answer is:
geyser
'water heater' is the definition.
(I've seen this before)
'nasty grey seen around home counties' is the wordplay.
'nasty' indicates an anagram (a nasty or wrong spelling).
'seen around' is an insertion indicator.
'home counties' becomes 'se' (Home Counties in the South East of England).
'grey' with letters rearranged gives 'geyr'.
'geyr' placed around 'se' is 'GEYSER'.
(Other definitions for geyser that I've seen before include "Steaming spring" , "A spring sending up columns of hot water" , "upsurge" , "Natural spout of hot water" , "It blows off steam" .)