Prevent endless travel sickness (5)
I believe the answer is:
avert
'prevent' is the definition.
(I know that avert is a type of prevent)
'endless travel sickness' is the wordplay.
'endless' says to take the centre.
'travel' becomes 'haj' (haj is a kind of travel).
'sickness' becomes 'vert' (I can't justify this - if you can you should give a lot more credence to this answer).
The middle of 'haj' is 'a'.
'a'+'vert'='AVERT'
Can you help me to learn more?
(Other definitions for avert that I've seen before include "Turn away (ones eyes)" , "Ward off (bad event)" , "Turn away from, say gaze" , "Deflect away" , "Turn aside, perhaps gaze" .)