Sinks in when couched in lower voice (6)
I believe the answer is:
basins
'sinks' is the definition.
(I've seen this before)
'when couched in lower voice' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'couched in' means one lot of letters goes inside another.
'lower voice' becomes 'bins' (I can't justify this - if you can you should believe this answer much more).
'as' placed into 'bins' is 'BASINS'.
'in' is the link.
Can you help me to learn more?
(Other definitions for basins that I've seen before include "Bowl-shaped vessels" , "Sinks" .)