He met extraordinary argument (5)
I believe the answer is:
theme
'argument' is the definition.
(I've seen this before)
'he met extraordinary' is the wordplay.
'extraordinary' is an anagram indicator.
'he'+'met'='hemet'
'hemet' with letters rearranged gives 'THEME'.
(Other definitions for theme that I've seen before include "(Writer's) topic" , "notedly subject to variation" , "Topical item" , "Melody on which variations are constructed" , "thing under discussion" .)