The box closes to capture sound (7)
I believe the answer is:
tenable
'sound' is the definition.
(both can mean solid or viable)
'the box closes to capture' is the wordplay.
'the box' becomes 'tele' (both can informally mean television).
'closes' indicates putting letters inside (inserted letters are closed or sealed in).
'to capture' becomes 'nab' (nab can mean to capture or arrest).
'tele' enclosing 'nab' is 'TENABLE'.
(Other definitions for tenable that I've seen before include "okay as regards defence" , "Of argument, rational and defensible" , "Capable of being kept" , "Late Ben (anag.)" , "Viable" .)