What there are in the paddock when one's clowning (6,6)

'what there' is the definition.
I can't judge whether this defines the answer.

'are in the paddock when one's clowning' is the wordplay.
I cannot quite understand how this works, but
'are' could be 'a' (short for 'are', historical unit of measurement) and 'a' is present in the answer.
'when' could be 'e' (I've seen this in other clues) and 'e' is found within the answer.
'one' could be 'un' (northern English dialect form of 'one') and 'un' is present in the answer.
This explanation may well be incorrect...

Can you help me to learn more?