In Peru cut short exciting trip once possibly (12)
I believe the answer is:
peradventure
'once possibly' is the definition.
(archaic word meaning 'possibly' or 'perhaps')
'in peru cut short exciting trip' is the wordplay.
'in' means one lot of letters goes inside another.
'peru' becomes 'pere' (I can't justify this - if you can you should give a lot more credence to this answer).
'cut short' means to remove the last letter.
'exciting trip' becomes 'adventure' (an adventure is an exciting trip).
'adventure' with its final letter taken away is 'adventur'.
'pere' enclosing 'adventur' is 'PERADVENTURE'.
(Other definitions for peradventure that I've seen before include "Possibly (archaic)" , "perhaps" , "Maybe" , "by chance" .)