Exploit computer key to copy both sides of advert (8)
I believe the answer is:
escapade
'exploit' is the definition.
(I've seen this before)
'computer key to copy both sides of advert' is the wordplay.
'computer key' becomes 'esc' (the Escape key on a keyboard).
'to copy' becomes 'ape' (synonyms).
'both sides of' means one lot of letters goes inside another (one word stretches round both sides of another).
'advert' becomes 'ad' (synonyms).
'ape' going around 'ad' is 'apade'.
'esc'+'apade'='ESCAPADE'
(Other definitions for escapade that I've seen before include "Adventurous incident" , "Jolly jape" , "Adventurous T [TREAT]" , "A reckless or mischievous adventure" , "Adventure, lark" .)