One chap you could flatten! (4)
I believe the answer is:
iron
'flatten' is the definition.
(I've seen this before)
'one chap' is the wordplay.
'one' becomes 'i' (Roman numeral).
'chap' becomes 'ron' (short for Ronald).
'i'+'ron'='IRON'
'you could' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for iron that I've seen before include "Strong metal; golf club" , "Fe - golf club" , "Very robust" , "Smooth - press - club" , "Revolver" .)