Took the seat beside one again, which is ironic (6)
I believe the answer is:
satire
'is ironic' is the definition.
I can't judge whether this definition defines the answer.
'took the seat beside one again' is the wordplay.
I cannot quite see how this works, but
'took' could be 'sat' (sit can mean to take an exam) and 'sat' is found in the answer.
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
'again' could be 're' (prefix meaning 'again') and 're' is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'which' acts as a link.
Can you help me to learn more?
(Other definitions for satire that I've seen before include "Lampoon" , "Unkind piece of writing" , "It holds up folly to ridicule" , "work with cutting devices?" , "Irony or sarcasm in speech or writing" .)