Rifle at first a boy keeps firing wickedly (5)
I believe the answer is:
arson
'firing wickedly' is the definition.
'arson' can be an answer for 'firing' (starting fires). I'm unsure of the 'wickedly' bit.
'rifle at first a boy keeps' is the wordplay.
'at first' indicates taking the first letters.
'a boy' becomes 'ason' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'keeps' indicates putting letters inside (inserted letters are kept inside).
The first letter of 'rifle' is 'r'.
'r' put inside 'ason' is 'ARSON'.
Can you help me to learn more?
(Other definitions for arson that I've seen before include "criminal fire-raising" , "matchplay, say?" , "Fire requiring forensic study" , "Felonious burning of property" , "Sonar (anag.)" .)