Travelling with pastry case, about to go (2,5)
I believe the answer is:
en route
'travelling' is the definition.
('en route' means on the way)
'with pastry case about to go' is the wordplay.
'with pastry case' becomes 'en croute' (in pastry crust).
'about' becomes 'c' (abbreviation for 'circa').
'to go' suggests deleting specific letters.
'encroute' with 'c' taken away is 'EN ROUTE'.
(Other definitions for en route that I've seen before include "In transit (French)" , "'On the way (2,5)'" , "Along the way" , "On the way - in France" , "March" .)