One hour's spent describing Meg's backside — cheeky! (7)
I believe the answer is:
roguish
'cheeky' is the definition.
(roguish can mean playfully cheeky)
'one hour's spent describing meg's backside' is the wordplay.
'one' becomes 'i' (Roman numeral).
'spent' is an anagram indicator.
'describing' is an insertion indicator (describe can mean to form the outline of).
'backside' indicates one should take the final letters (the letter at the back).
The final letter of 'meg' is 'g'.
'i'+'hours'='ihours'
'ihours' with letters rearranged gives 'rouish'.
'rouish' placed around 'g' is 'ROGUISH'.
(Other definitions for roguish that I've seen before include "Coy" , "Saucy" , "Mischievous; unprincipled" , "Playfully mischievous" , "reprobate" .)