The end of the third round - one is flattened (6)
I believe the answer is:
ironed
'is flattened' is the definition.
(I've seen this before)
'the end of the third round one' is the wordplay.
I cannot really see how this works, but
an anagram of 'end' is 'ned' which is present in the answer.
'of' could be 'o' and 'o' is present in the answer.
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
A single letter 'r' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for ironed that I've seen before include "prepared for 23 [WARDROBE] ?" , "Got rid of creases" , "Smoothed out wrinkles" , "Given a press" , "Pressed and smoothed" .)