Unconventional leading character — one in another form (8)
I believe the answer is:
antihero
'unconventional leading character' is the definition.
'antihero' can be an answer for 'character' (antihero is a kind of character). I'm unsure of the 'unconventional leading' bit.
'one in another form' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'form' is an anagram indicator (a different form of the same letters).
'another' is an anagram of 'anthero'.
'i' put into 'anthero' is 'ANTIHERO'.
Can you help me to learn more?
(Other definitions for antihero that I've seen before include "central character, say" , "Character such as Meursault" , "ignoble character" , "Unconventional principal character" , "principal character lacking noble qualities" .)