Instrument requiring twice as sound foundation (6,4)
I believe the answer is:
double bass
'instrument' is the definition.
(I know that double bass is a type of bass)
'twice as sound foundation' is the wordplay.
'twice as' becomes 'double' (synonyms).
'sound' indicates a 'sounds like' (homophone) clue.
'foundation' becomes 'base' (synonyms).
'base' is a homophone of 'bass'.
'double'+'bass'='DOUBLE-BASS'
'requiring' acts as a link.
(Other definitions for double bass that I've seen before include "Stringed instrument" , "... as part of orchestra" , "One taking a bow" , "Musical instrument" , "Low-pitched string instrument" .)