Leads from random encounter could change entire investigation (5)
I believe the answer is:
recce
'investigation' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for recce that I've seen before include "Advance exploration" , "look round" , "Survey (sl.)" , "Military observation" , "Inspection" .)