Just twenty letters - not much put on top of what's taken from bin ... (3)
I believe the answer is:
tot
'just twenty letters not much put' is the definition.
I can't tell whether this definition defines the answer.
'on top of what's taken from bin' is the wordplay.
'on top' becomes 'too' (I've seen this before).
'of' becomes 'o'.
'what's taken from' indicates named letters should be taken away.
'bin' becomes 't' (I can't justify this - if you can you should believe this answer much more).
'too' with 'o' taken out is 'to'.
'to'+'t'='TOT'
Can you help me to learn more?
(Other definitions for tot that I've seen before include "Small drink; accumulate" , "Aggregate for infant" , "Party drink" , "Palindrome for small child" , "Small baby" .)