Compound very like another is more unstable (6)
I believe the answer is:
isomer
'compound very like another' is the definition.
(compound with the same formula as another)
'is more unstable' is the wordplay.
'unstable' indicates an anagram.
'more' is an anagram of 'omer'.
'is'+'omer'='ISOMER'
(Other definitions for isomer that I've seen before include "Compound with same atoms as another but different structure" , "similar substance" , "Compound sharing a formula with another" , "Compound of identical atoms to another but in differing arrangement" , "One of different compounds of same formula" .)