Call out "Rich has a gun!" (8)
I believe the answer is:
christen
'call' is the definition.
(christening is a kind of calling)
'out rich has a gun' is the wordplay.
'out' indicates anagramming the letters.
'has' is a charade indicator (letters next to each other).
'a gun' becomes 'sten' (British sub-machine gun).
'rich' is an anagram of 'chri'.
'chri'+'sten'='CHRISTEN'
(Other definitions for christen that I've seen before include "Use for the first time" , "Give a name to (baby)" , "Baptise and name" , "Baptize" , "Name in church" .)