You can count on it (6)
'you can count on it' is the definition.
(an abacus is a tool for doing sums)
'you can count on' is the wordplay.
'you' becomes 'u' (commonly used in online communication).
'can count' becomes 'abacs' (I can't explain this - if you can you should believe this answer much more).
'on' is an insertion indicator (as in clothing 'on' a person).
'u' put into 'abacs' is 'ABACUS'.
Can you help me to learn more?
(Other definitions for abacus that I've seen before include "An old counting frame with beads" , "What was counted on" , "As a cub (anag)" , "this adds up" , "not something we now count on" .)
(Other definitions for digit that I've seen before include "a number less than ten" , "that's on hand" , "finger, perhaps" , "Finger; number" , "A small number" .)
