Book subsequently incorporating southern explosives expert (7)
I believe the answer is:
blaster
'explosives expert' is the definition.
(someone who blasts things)
'book subsequently incorporating southern' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'subsequently' becomes 'later' (subsequently means later on).
'incorporating' is an insertion indicator.
'southern' becomes 's' (abbreviation).
'later' going around 's' is 'laster'.
'b'+'laster'='BLASTER'
(Other definitions for blaster that I've seen before include "Creator of an explosion" , "Wedge" , "Quarryman with explosive?" .)