Danger: it's unstable star! (3,5)
I believe the answer is:
red giant
'star' is the definition.
(I've seen this before)
'danger it's unstable' is the wordplay.
'unstable' indicates an anagram.
'danger'+'it'='dangerit'
'dangerit' with letters rearranged gives 'RED GIANT'.
(Other definitions for red giant that I've seen before include "Huge Communist" , "big star" , "Large low-temperature star" , "Type of star" , "Cool, immense star of high luminosity" .)