There are three in there, four in that one and six in my pocket (10)
I believe the answer is:
consonants
'there are three in there four in' is the definition.
I can't tell whether this definition defines the answer.
'one and six in my pocket' is the wordplay.
I cannot really understand how this works, but
'one' could be 'an' (an apple is one apple) and 'an' is found within the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'that' is the link.
Can you help me to learn more?
(Another definition for consonants that I've seen is " Letters".)