These initially facilitate sound (7)
I believe the answer is:
tenable
'sound' is the definition.
(both can mean solid or viable)
'these initially facilitate' is the wordplay.
'initially' says to take the initial letters.
'facilitate' becomes 'enable' (I've seen this in another clue).
The first letter of 'these' is 't'.
't'+'enable'='TENABLE'
(Other definitions for tenable that I've seen before include "Believable" , "Able to be held" , "Possible to be defended" , "fairly arguable" , "Late Ben (anag.)" .)