Biographer's material, what's known of it? (3,5,2,4)

'biographer's material what's known' is the definition.
I can't judge whether this defines the answer.

'of it?' is the wordplay.
I cannot really see how this works, but
'of' could be 'o' and 'o' is found within the answer.
'it?' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
This explanation may well be incorrect...