Fighter regularly gauged gunners around trench (9)
I believe the answer is:
guerrilla
'fighter' is the definition.
(as in guerrilla warfare)
'regularly gauged gunners around trench' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'gunners' becomes 'RA' (Royal Artillery).
'around' indicates putting letters inside.
'trench' becomes 'rill' (I can't explain this - if you can you should give a lot more credence to this answer).
The alternating letters of 'gauged' are 'gue'.
'ra' going around 'rill' is 'rrilla'.
'gue'+'rrilla'='GUERRILLA'
Can you help me to learn more?
(Other definitions for guerrilla that I've seen before include "Underground fighter" , "Person who fights as one of small independent group" , "Partisan" , "Resistance fighter" , "Combatant" .)