Second caution causes alarm (5)
I believe the answer is:
scare
'alarm' is the definition.
(I've seen this before)
'second caution' is the wordplay.
'second' becomes 's' (common abbreviation).
'caution' becomes 'care' ('care' can be a synonym of 'caution').
's'+'care'='SCARE'
'causes' acts as a link.
(Other definitions for scare that I've seen before include "Cause fright, alarm" , "Panic" , "Startle" , "Greatly frighten" , "Frighten, alarm" .)