Additional outlet, one no longer used by gunners (5)
I believe the answer is:
extra
'additional' is the definition.
(extra means additional or surplus)
'outlet one no longer used by gunners' is the wordplay.
'outlet one' becomes 't' (I am not sure about this - if you are sure you should believe this answer much more).
'no longer' becomes 'ex' (prefix meaning former or onetime).
'used by' is a charade indicator (letters next to each other).
'gunners' becomes 'RA' (Royal Artillery).
't' put after 'ex' is 'ext'.
'ext'+'ra'='EXTRA'
Can you help me to learn more?
(Other definitions for extra that I've seen before include "Additional - bye" , "more than usually" , "Actor's part" , "Bit actor in movie's crowd scenes" , "Minor actors in crowd scenes" .)