Irrational character had gone wrong in the topic (3,6)
I believe the answer is:
mad hatter
'irrational character' is the definition.
(character in Alice's Adventures in Wonderland)
'had gone wrong in the topic' is the wordplay.
'gone wrong' indicates anagramming the letters.
'in the' means one lot of letters goes inside another.
'topic' becomes 'matter' (synonyms).
'had' is an anagram of 'adh'.
'adh' going inside 'matter' is 'MAD HATTER'.
(Other definitions for mad hatter that I've seen before include "Participant in the 16 in 3,8" , "one forever taking tea" , "One at party" , "attending a literary tea party" , "Eccentric" .)