Declare time to head off (5)
I believe the answer is:
avert
'head off' is the definition.
(synonyms)
'declare time' is the wordplay.
'declare' becomes 'aver' (averring is a kind of declaring).
'time' becomes 't'.
'aver'+'t'='AVERT'
'to' is the link.
(Other definitions for avert that I've seen before include "Turn away (ones eyes)" , "Ward off (bad event)" , "Head off" , "Turn away, prevent" , "Turn aside, prevent" .)