Superlative adverb (4)
I believe the answer is:
most
'superlative adverb' is the definition.
(I've seen this in another clue)
This is the entire clue.
(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" .)