It’s wholly associated with caboodle (3)
I believe the answer is:
kit
'caboodle' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'it's wholly associated with' is the wordplay.
'it's' becomes 'it' (I've seen this in other clues).
'wholly' becomes 'k' (I am not sure about this - if you are sure you should believe this answer much more).
'associated with' means one lot of letters go next to another.
'it' after 'k' is 'KIT'.
Can you help me to learn more?
(Other definitions for kit that I've seen before include "Football top, shorts, stockings and boots?" , "Apparatus" , "Stuff" , "Young stoat" , "Equipment; young ferret" .)