Become prone to show absolute support for fiction... (3,4)

'become' is the definition.
I don't know anything about this answer so I can't judge whether this works.

'prone to show absolute support for fiction' is the wordplay.
I cannot quite understand how this works, but
'prone' could be 'flat' (I've seen this in another clue) and 'flat' is found in the answer.
'fiction' could be 'lie' (fiction can mean a lie or fabrication) and 'lie' is found within the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?