Ring number of US soldiers up (6)
I believe the answer is:
signet
'ring' is the definition.
(signet ring is a kind of ring)
'number of us soldiers up' is the wordplay.
'number of' becomes 'ten' (I can't explain this - if you can you should believe this answer much more).
'us soldiers' becomes 'gis' (GI, nickname for US soldier).
'up' is a reversal indicator (in down clue: letters go upwards).
'ten'+'gis'='tengis'
'tengis' in reverse letter order is 'SIGNET'.
Can you help me to learn more?
(Other definitions for signet that I've seen before include "Small seal, on ring perhaps" , "Official seal" , "Small seal set in a ring" , "Small seal sounds like a young swan" , "Tuft of hanging threads" .)