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' is an anagram indicator (letters developed into a new form).
'on' is a charade indicator (letters next to each other).
'time' becomes 't' (abbreviation).
'rave' is an anagram of 'aver'.
'aver'+'t'='AVERT'
(Other definitions for avert that I've seen before include "Prevent from happening" , "Deflect away" , "Avoid" , "Stop" , "Turn aside, prevent" .)