Mate in panic: moon exploding (9)
I believe the answer is:
companion
'mate' is the definition.
(thesaurus)
'panic moon exploding' is the wordplay.
'exploding' is an anagram indicator.
'panic'+'moon'='panicmoon'
'panicmoon' is an anagram of 'COMPANION'.
'in' is the link.
(Other definitions for companion that I've seen before include "Mate (maybe of honour!)" , "Intimate" , "Person who lives or travels with you" , "Frequent associate" , "person with whom one is travelling?" .)