What’s irregular or oddly regular about one line? (9)
I believe the answer is:
guerrilla
'what's irregular' is the definition.
(synonyms)
'regular about one line?' is the wordplay.
'about' indicates anagramming the letters.
'one' becomes 'i' (Roman numeral).
'line?' becomes 'l' (used when specifying particular lines from a poem).
'i'+'l'='il'
'regular' anagrammed with 'il' gives 'GUERRILLA'.
'or oddly' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for guerrilla that I've seen before include "Resistance fighter" , "Underground fighter" , "political insurgent" , "Member of politically-motivated fighting group" , "One fighting" .)