Evangelist affable? Not that interested (8)
I believe the answer is:
lukewarm
'not that interested' is the definition.
(I know that lukewarm is a more specific form of the attribute not that interested)
'evangelist affable?' is the wordplay.
'evangelist' becomes 'luke' (one of the four Christian evangelists).
'affable?' becomes 'warm' (warm can mean friendly or affable).
'luke'+'warm'='LUKEWARM'
(Other definitions for lukewarm that I've seen before include "Not very keen" , "Unenthusiastic" , "Tepid, or less than enthusiastic" , "Moist" , "Somewhat cooler!" .)