Novel exploring manners mostly, Austen's first of all (4)
I believe the answer is:
emma
'novel' is the definition.
(I've seen this before)
'exploring manners mostly austen's first of all' is the wordplay.
'first of all' indicates taking the first letters.
The first letters of 'exploring manners mostly austen' is 'EMMA'.
(Other definitions for emma that I've seen before include "Austen character" , "Miss Woodhouse?" , "Eponymous heroine" , "Title of novel by Jane Austen" , "young woman in novel" .)