Tough concealing dope while travelling (2,7)
I believe the answer is:
in transit
'while travelling' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for in transit that I've seen before include "While being conveyed" , "during journey" , "Being transported" , "En route" , "On its way" .)