He introduces politician before going after commanding officer (7)
I believe the answer is:
compere
'he introduces' is the definition.
'politician before going after commanding officer' is the wordplay.
'politician' becomes 'mp'.
'before' becomes 'ere' (usually literary).
'going after' says to put letters next to each other.
'commanding officer' becomes 'co'.
'mp'+'ere'='mpere'
'mpere' put after 'co' is 'COMPERE'.
(Other definitions for compere that I've seen before include "TV presenter?" , "host show" , "(Male) show introducer" , "Master of ceremonies" , "M.C." .)