Reason to observe rising smoke (5)
I believe the answer is:
logic
'reason' is the definition.
(logic is using reason)
'observe rising smoke' is the wordplay.
'observe' becomes 'lo' (archaic exclamation meaning 'look!').
'rising' is a reversal indicator (going up in a down clue).
'smoke' becomes 'cig' (cigarette).
'cig' backwards is 'gic'.
'lo'+'gic'='LOGIC'
'to' acts as a link.
(Other definitions for logic that I've seen before include "Branch of philosophy that analyses inference" , "Aristotelian theories" , "good sense" , "Reasoned argument" , "Science of correct reasoning" .)