Huge amount of slime on backed-up register (6)
I believe the answer is:
googol
'huge amount' is the definition.
The definition suggests a singular noun which matches the answer.
'slime on backed-up register' is the wordplay.
'slime' becomes 'goo' (synonyms).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'backed up' says the letters should be written in reverse.
'register' becomes 'log' (a log is a register).
'log' written backwards gives 'gol'.
'goo'+'gol'='GOOGOL'
'of' is the link.
Can you help me to learn more?
(Other definitions for googol that I've seen before include "unimaginable figure" , "10100" , "Big number" , "very large number" , "Vast figure" .)