Massive group from NATO left on the road (5)
I believe the answer is:
horde
'massive group' is the definition.
(I've seen this in another clue)
'from nato left on the road' is the wordplay.
'from' indicates an anagram (from the letters).
'left' is a deletion indicator.
'on'+'the'+'road'='ontheroad'
'ontheroad' with 'nato' removed is 'herod'.
'herod' anagrammed gives 'HORDE'.
(Other definitions for horde that I've seen before include "Large force" , "Large crowd" , "Migratory tribe; army" , "Nomadic army" , "A moving crowd" .)