The majority are used in thermostats (4)
I believe the answer is:
most
'the majority' is the definition.
(most of a group)
'used in thermostats' is the wordplay.
'used in' indicates a hidden word.
'MOST' can be found hidden inside 'thermostats'.
'are' acts as a link.
(Other definitions for most that I've seen before include "Almost all" , "Kind of commercial pact where two countries agree to treat each other as well as any other" , "Greatest in amount or degree" , "In the highest degree" , "Nearly all, majority" .)