Maybe a mite cheeky, keeping a rumpus up (8)
I believe the answer is:
arachnid
'maybe a mite' is the definition.
'cheeky keeping a rumpus up' is the wordplay.
'cheeky' becomes 'arch'.
'keeping' means one lot of letters goes inside another.
'rumpus' becomes 'din' ('din' can be a synonym of 'rumpus').
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'din' back-to-front is 'nid'.
'arch' placed around 'a' is 'arach'.
'arach'+'nid'='ARACHNID'
(Other definitions for arachnid that I've seen before include "Spider or scorpion for example" , "Incy Wincy Spider, say" , "One of species including spiders, scorpions etc" , "Member of the spider family" , "tick for one" .)