Flee, pocketing diamonds after a mischievous adventure (8)
I believe the answer is:
escapade
'adventure' is the definition.
('escapade' can be a synonym of 'adventure')
'flee pocketing diamonds after a mischievous' is the wordplay.
'flee' becomes 'escape' (escaping is a kind of fleeing**).
'pocketing' means one lot of letters goes inside another.
'diamonds' becomes 'd' (abbreviation in bridge say).
'after' says to put letters next to each other.
'mischievous' indicates an anagram.
'd'+'a'='da'
'da' with letters rearranged gives 'ad'.
'escape' enclosing 'ad' is 'ESCAPADE'.
(Other definitions for escapade that I've seen before include "quite a lark" , "Wild adventure" , "Daring stunt" , "Mischievous exploit" , "Madcap adventure" .)