Monumental queues forming behind National, endlessly (10)
I believe the answer is:
statuesque
'monumental' is the definition.
(thesaurus)
'queues forming behind national endlessly' is the wordplay.
'forming' indicates an anagram.
'behind' is a charade indicator (letters next to each other).
'national endlessly' becomes 'stat' (I am not sure about this - if you are sure you should believe this answer much more).
'queues' is an anagram of 'uesque'.
'uesque' after 'stat' is 'STATUESQUE'.
(Other definitions for statuesque that I've seen before include "(Of a woman) tall and dignified" , "imposing" .)