Army hospital with tidiness falling short (5)
I believe the answer is:
horde
'army' is the definition.
(I've seen this before)
'hospital with tidiness falling short' is the wordplay.
'hospital' becomes 'H' (abbreviation for hospital).
'with' says to put letters next to each other.
'tidiness' becomes 'order' (I've seen this in another clue).
'falling short' means to remove the last letter.
'order' with its final letter taken away is 'orde'.
'h'+'orde'='HORDE'
(Other definitions for horde that I've seen before include "Migratory tribe; army" , "Large army" , "Nomadic army" , "Large crowd" , "Huge noisy crowd" .)