Opening part of travel permit that's sound (7)
I believe the answer is:
tenable
'sound' is the definition.
(both can mean solid or viable)
'opening part of travel permit' is the wordplay.
'opening part of' says to take the initial letters.
'permit' becomes 'enable' (I've seen this in another clue).
The first letter of 'travel' is 't'.
't'+'enable'='TENABLE'
'that's' acts as a link.
(Other definitions for tenable that I've seen before include "Able to be believed" , "Of argument, rational and defensible" , "Based on sound reasoning or evidence" , "Viable" , "okay as regards defence" .)