Thickness of book perused by author, essentially (7)
I believe the answer is:
breadth
'thickness' is the definition.
(thesaurus)
'book perused by author essentially' is the wordplay.
'book' becomes 'b'.
'perused' becomes 'read' (I've seen this before).
'by' means one lot of letters go next to another.
'essentially' says to take the centre.
The middle letters of 'author' are 'th'.
'b'+'read'+'th'='BREADTH'
'of' acts as a link.
(Other definitions for breadth that I've seen before include "Wide measurement" , "Measure" , "Wideness" , "Liberality" , "Measurement from side to side" .)