Provoke retired pilot to assess penning book (10)
I believe the answer is:
exacerbate
'provoke' is the definition.
(provoke can mean to exacerbate or make worse)
'retired pilot to assess penning book' is the wordplay.
'retired' becomes 'ex' (prefix meaning former or onetime).
'pilot' becomes 'ace' (term for military pilot).
'to assess' becomes 'rate' (rating is a kind of assessing).
'penning' means one lot of letters goes inside another.
'book' becomes 'b' (abbreviation for book).
'rate' placed around 'b' is 'rbate'.
'ex'+'ace'+'rbate'='EXACERBATE'
(Other definitions for exacerbate that I've seen before include "Provoke" , "Make (a bad thing) worse" , "Worsen" .)