Three become solvent (5)
I believe the answer is:
ether
'solvent' is the definition.
(thesaurus)
'three become' is the wordplay.
'become' indicates an anagram.
'three' with letters rearranged gives 'ETHER'.
(Other definitions for ether that I've seen before include "Highly flammable liquid once used as an anaesthetic" , "Volatile solvent" , "Solvent; clear sky (poet.)" , "Anaesthetic; clear sky (poetic)" , "Rare atmosphere" .)