Narrow escape as armies moving east to west within nanoseconds (4,4)
I believe the answer is:
near miss
'narrow escape' is the definition.
(thesaurus)
'armies moving east to west within nanoseconds' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'armies' is 'earmis' which is present in the answer.
'nanoseconds' could be 'ns' (abbreviation) and 'ns' is present in the remaining letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'as' acts as a link.
(Other definitions for near miss that I've seen before include "Attempt that just fails to be successful" , "Failure just" , "Close thing" , "Narrowly-averted accident" , "almost getting killed?" .)