Singular cash register is quiet (5)
I believe the answer is:
still
'quiet' is the definition.
(I know that quiet can be written as still)
'singular cash register' is the wordplay.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'cash register' becomes 'till' (I've seen this before).
's'+'till'='STILL'
'is' is the link.
(Other definitions for still that I've seen before include "Source of alcohol" , "Snap" , "Silence" , "activity of one manufacturing spirits?" , "Calm, unmoving" .)