The boy Derek, about six, a mischievous imp (5)
I believe the answer is:
devil
'a mischievous imp' is the definition.
'devil' can be an answer for 'imp' (I've seen this in another clue). I'm not certain of the 'a mischievous' bit.
'the boy derek about six' is the wordplay.
'the boy derek' becomes 'del' (I can't justify this - if you can you should give a lot more credence to this answer).
'about' is an insertion indicator.
'six' becomes 'VI' (Roman numerals).
'del' going around 'vi' is 'DEVIL'.
Can you help me to learn more?
(Other definitions for devil that I've seen before include "Old Nick, Satan" , "Demon" , "Coat spicily; do hackwork" , "See 11" , "Imp" .)