Be suspicious of vapour with reddish-brown colour (8)
I believe the answer is:
mistrust
'be suspicious of' is the definition.
'vapour with reddish-brown colour' is the wordplay.
'vapour' becomes 'mist' (water vapour).
'with' is a charade indicator (letters next to each other).
'reddish-brown colour' becomes 'rust' (I've seen this before).
'mist'+'rust'='MISTRUST'
(Other definitions for mistrust that I've seen before include "Suspect (someone)" , "Have no confidence in (someone)" , "reservations" , "Lack of belief (in someone)" , "Have doubts about" .)