Robber's first into cash register, getting high-pitched sound (5)
I believe the answer is:
trill
'high-pitched sound' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Maybe you can see a link between them that I can't see?
'robber's first into cash register' is the wordplay.
'robber's first' becomes 'r' (1st letter of 'robber').
'into' means one lot of letters goes inside another.
'cash register' becomes 'till' (I've seen this before).
'r' going into 'till' is 'TRILL'.
'getting' acts as a link.
Can you help me to learn more?
(Other definitions for trill that I've seen before include "Sound of rapidly alternating notes" , "Shake" , "sing" , "Fast alternation of notes" , "Rapid alternation of notes" .)