Prevent bad smell coming up through hollow tube (7)
I believe the answer is:
obviate
'prevent' is the definition.
(I know this)
'bad smell coming up through hollow tube' is the wordplay.
'bad smell' becomes 'bo' (informal abbreviation for body odour).
'coming up' is a reversal indicator (in a down clue, letters go up).
'through' becomes 'via' (via means going through).
'hollow' suggests removing the centre.
'tube' with its middle taken out is 'te'.
'bo' backwards is 'ob'.
'ob'+'via'+'te'='OBVIATE'
(Other definitions for obviate that I've seen before include "Counter" , "Get round inconvenience - get above it" , "Get around or do away with, say inconvenience" , "Preclude" , "Avoid, prevent" .)