Extremist gunmen tailing superior officer (5)
I believe the answer is:
ultra
'extremist' is the definition.
(synonyms)
'gunmen tailing superior officer' is the wordplay.
'gunmen' becomes 'RA' (Royal Artillery).
'tailing' is a charade indicator (letters next to each other) (to tail can mean to go after or follow).
'superior' becomes 'u' (I've seen this in other clues).
'officer' becomes 'lt' (I've seen this in other clues).
'u'+'lt'='ult'
'ra' after 'ult' is 'ULTRA'.
(Other definitions for ultra that I've seen before include "Prefix meaning beyond or extremely" , "Over the top" , "Far beyond the norm" , "Extreme - extremist" , "Prefix meaning extreme or excessive" .)