Mischief-makers: one thousand, what's more ... (4)
I believe the answer is:
imps
'mischief-makers' is the definition.
(I've seen this in another clue)
'one thousand what's more' is the wordplay.
'one' becomes 'i' (Roman numeral).
'thousand' becomes 'mps' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'what's more' means one lot of letters go next to another (some letters added to the previous ones).
'i'+'mps'='IMPS'
Can you help me to learn more?
(Other definitions for imps that I've seen before include "Mischievous spirits" , "Little devils or scamps" , "Little monkeys?" , "naughty children" , "Mischievous fairies" .)