Pay about the top of sixty-six billion, to go unnoticed (4,2)
I believe the answer is:
pass by
'go unnoticed' is the definition.
I can't judge whether this defines the answer.
'pay about the top of sixty-six billion' is the wordplay.
'about' indicates putting letters inside.
'the top of' suggests taking the first letters.
The initial letters of 'sixty six billion' is 'ssb'.
'pay' going around 'ssb' is 'PASS BY'.
'to' is the link.
Can you help me to learn more?
(Other definitions for pass by that I've seen before include "Go past - disregard" , "Leave on one side" , "''Cast a cold eye / On life, on death / Horseman ... ... (WB Yeats)" , "Ignore, overlook" , "neglect" .)