Rifle poorly held by knight’s assistant (7)
I believe the answer is:
pillage
'rifle' is the definition.
(both can mean to rob a place)
'poorly held by knight's assistant' is the wordplay.
'poorly' becomes 'ill' (similar in meaning).
'held by' means one lot of letters goes inside another.
'knight's assistant' becomes 'page' (I can't explain this - if you can you should give a lot more credence to this answer).
'ill' placed within 'page' is 'PILLAGE'.
Can you help me to learn more?
(Other definitions for pillage that I've seen before include "Violent stealing" , "Rob with violence" , "Sacking" , "Plunder, sack" , "Loot" .)