Cash registers (5)
I believe this is a double definition.
'cash' is the first definition.
(as in banknotes)
'registers?' is the second definition.
(both can mean to notice or observe)
(Other definitions for notes that I've seen before include "Memoranda" , "Brief letters; money" , "Stone with jottings" , "Pays attention" , "Written comments" .)
'cash registers' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for tills that I've seen before include "Cash registers" , "Ploughs land" .)