Many lacking in caution collide (5)
I believe the answer is:
crash
'collide' is the definition.
(I know that crash is a more specific form of the action collide)
'many lacking in caution' is the wordplay.
'many' becomes 'c' (C is the Roman numeral for 100).
'lacking in caution' becomes 'rash' (I've seen this before).
'c'+'rash'='CRASH'
(Other definitions for crash that I've seen before include "failure" , "Disastrous market fall" , "Dramatic fall" , "Impact" , "Economic collapse" .)