Best part of a thermostat (4)
I believe the answer is:
most
'best part' is the definition.
(thesaurus)
'of a thermostat' is the wordplay.
'of' says the answer is hidden in the clue.
'MOST' can be found hidden inside 'thermostat'.
(Other definitions for most that I've seen before include "Almost all" , "Nearly all, majority" , "Kind of commercial pact where two countries agree to treat each other as well as any other" , "Extremely, very" , "Greatest in amount or degree" .)