Marines manoeuvring south narrowly avoided collision (4,4)
I believe the answer is:
near miss
'narrowly avoided collision' is the definition.
(near miss can mean a narrowly avoided crash)
'marines manoeuvring south' is the wordplay.
'manoeuvring' indicates an anagram.
'south' becomes 'S' (abbreviation).
'marines' with letters rearranged gives 'nearmis'.
'nearmis'+'s'='NEAR MISS'
(Other definitions for near miss that I've seen before include "Wide of the target in seminars" , "Failure just" , "Almost a collision" , "Not quite on target, but close" , "Narrowly-averted accident" .)