Leonard is not worried about being cheeky (8)
I believe the answer is:
insolent
'cheeky' is the definition.
(I've seen this before)
'leonard is not worried about' is the wordplay.
'leonard' becomes 'len' (I've seen this in other clues).
'worried' indicates anagramming the letters.
'about' indicates putting letters inside.
'is'+'not'='isnot'
'isnot' with letters rearranged gives 'insot'.
'len' inserted into 'insot' is 'INSOLENT'.
'being' acts as a link.
(Other definitions for insolent that I've seen before include "Pert" , "Lippy" , "Impertinent" , "Saucy" , "Rude and disrespectful" .)