Mischievous activities for early risers (5)
I believe the answer is:
larks
'mischievous activities' is the definition.
'larks' can be an answer for 'activities' (lark is a kind of activity). I am not sure about the 'mischievous' bit.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for larks that I've seen before include "Small songbirds" , "Fun and games" , "Frolics" , "Early birds" , "High jinks" .)