See about new exhaust? Absolutely! (8)
I believe the answer is:
entirely
'absolutely' is the definition.
(synonyms)
'see about new exhaust?' is the wordplay.
'see' becomes 'ely' (see is a place with a cathedral - e.g. Ely).
'about' is an insertion indicator.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'exhaust?' becomes 'tire' ('tire' can be a synonym of 'exhaust').
'n'+'tire'='ntire'
'ely' going around 'ntire' is 'ENTIRELY'.
(Other definitions for entirely that I've seen before include "Altogether" , "'Fully, wholly (8)'" , "Completely and absolutely" , "Without reservation" , "Wholly, completly" .)