Prevent rave developing on time (5)
I believe the answer is:
avert
'prevent' is the definition.
(averting is a kind of preventing)
'rave developing on time' is the wordplay.
'developing' indicates anagramming the letters.
'on' means one lot of letters go next to another.
'time' becomes 't'.
'rave' anagrammed gives 'aver'.
'aver'+'t'='AVERT'
(Other definitions for avert that I've seen before include "Prevent from happening" , "Deflect away" , "Avoid" , "Stop" , "Turn aside, prevent" .)