US author has quirk that makes him rhythmical (6)
I believe the answer is:
poetic
'rhythmical' is the definition.
(I've seen this before)
'us author has quirk' is the wordplay.
'us author' becomes 'Poe' (Edgar Allan Poe).
'has' says to put letters next to each other.
'quirk' becomes 'tic' (I've seen this in another clue).
'poe'+'tic'='POETIC'
'that makes him' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for poetic that I've seen before include "Written in verse" , "like Byron's" , "Ice pot (anag.)" , "One sort of licence" , "Expressed in verse" .)