Month accompanying a star (4)
I believe the answer is:
nova
'star' is the definition.
(I know that nova is a type of star)
'month accompanying a' is the wordplay.
'month' becomes 'nov' (abbreviation for November).
'accompanying' is a charade indicator (letters next to each other).
'nov'+'a'='NOVA'
(Other definitions for nova that I've seen before include "It shines" , "Suddenly expanding star" , "Star showing a sudden temporary increase in brightness" , "Suddenly bright star" , "Newly bright star" .)