Exciting experience after agent returned by chance (12)
I believe the answer is:
peradventure
'by chance' is the definition.
(I've seen this before)
'exciting experience after agent returned' is the wordplay.
'exciting experience' becomes 'adventure' (I've seen this before).
'after' says to put letters next to each other.
'agent' becomes 'rep' (short for representative).
'returned' shows that the letters should be reversed in order.
'rep' backwards is 'per'.
'adventure' after 'per' is 'PERADVENTURE'.
(Other definitions for peradventure that I've seen before include "by chance" , "Possibly (archaic)" , "Maybe" , "perhaps" .)