Signs from unstable meson (5)
I believe the answer is:
omens
'signs' is the definition.
(omen is a kind of sign)
'unstable meson' is the wordplay.
'unstable' indicates an anagram.
'meson' anagrammed gives 'OMENS'.
'from' acts as a link.
(Other definitions for omens that I've seen before include "Signs of things to come" , "Auguries" , "Harbingers, portents" , "Signs or warnings for the future" , "Augurs" .)