Spark is appropriate around Scottish chimney, on reflection (8)
I believe the answer is:
stimulus
'spark' is the definition.
'is appropriate around scottish chimney on reflection' is the wordplay.
'is appropriate' becomes 'suits' (to suit is to be appropriate for).
'around' means one lot of letters goes inside another.
'scottish chimney' becomes 'lum' (Scottish term for a chimney).
'on reflection' shows that the letters should be reversed in order.
'suits' placed around 'lum' is 'sulumits'.
'sulumits' written backwards gives 'STIMULUS'.
(Other definitions for stimulus that I've seen before include "Motivating action" , "Cause of a response" , "Goad" , "Something provoking a response" , "Arousing thing?" .)