More than half is used in thermostats (4)
I believe the answer is:
most
'more than half' is the definition.
(I've seen this in another clue)
'used in thermostats' is the wordplay.
'used in' indicates a hidden word.
'MOST' is hidden within 'thermostats'.
'is' is the link.
(Other definitions for most that I've seen before include "Greatest in amount or degree" , "Kind of commercial pact where two countries agree to treat each other as well as any other" , "In the highest degree" , "Almost all" , "Extremely, very" .)