Characteristic to conceal occasionally one's route (7)
I believe the answer is:
transit
'route' is the definition.
(en route means in transit)
'characteristic to conceal occasionally one's' is the wordplay.
'characteristic' becomes 'trait' (a trait is a quality or characteristic).
'to conceal' means one lot of letters goes inside another.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
The alternate letters of 'ones' are 'ns'.
'trait' placed around 'ns' is 'TRANSIT'.
(Other definitions for transit that I've seen before include "To pass through/across" , "Passage of heavenly body" , "In - - (en route)" , "Movement from place to place" , "Start in for the process of being conveyed" .)