Smell comes from small channel, not river (6)
I believe the answer is:
stench
'smell' is the definition.
(I know that smell can be written as stench)
'small channel not river' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'channel' becomes 'trench' (I've seen this before).
'not' indicates named letters should be taken away.
'river' becomes 'r' (abbreviation for river).
'trench' with 'r' removed is 'tench'.
's'+'tench'='STENCH'
'comes from' acts as a link.
(Other definitions for stench that I've seen before include "Strong unpleasant smell" , "Miasma" , "A really foul smell" , "Horrid smell" , "Awful smell" .)