Innumerable other ranks including private following uniform (7)
I believe the answer is:
'innumerable' is the definition.
(I've seen this in another clue)
'other ranks including private following uniform' is the wordplay.
'other ranks' becomes 'men' (soldiers who are not commissioned officers).
'including' is an insertion indicator.
'private' becomes 'pte'.
'following' is a charade indicator (letters next to each other).
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'men' going around 'pte' is 'mpteen'.
'mpteen' put after 'u' is 'UMPTEEN'.
(Other definitions for umpteen that I've seen before include "Large but indefinite number" , "Countless" , "An uncountable number" , "Numerous" , "Guarded" .)