Bill hiding in skip — such a lark! (8)
I believe the answer is:
escapade
'such a lark' is the definition.
(both can mean an adventure or amusement)
'bill hiding in skip' is the wordplay.
'bill' becomes 'ad' (advertisement stuck on wall).
'hiding in' indicates putting letters inside.
'skip' becomes 'escape' (skip can mean to leave or escape).
'ad' going into 'escape' is 'ESCAPADE'.
(Other definitions for escapade that I've seen before include "Antic" , "Adventurous incident" , "Madcap adventure" , "A reckless or mischievous adventure" , "Mischievous romp" .)