Sovereign's upset amateur soldiers (5)
I believe the answer is:
royal
'sovereign's' is the definition.
(I've seen this in another clue)
'upset amateur soldiers' is the wordplay.
'upset' is a reversal indicator.
'amateur' becomes 'lay' (similar in meaning).
'soldiers' becomes 'OR' (military abbreviation for other ranks).
'lay'+'or'='layor'
'layor' in reverse letter order is 'ROYAL'.
(Other definitions for royal that I've seen before include "Queen, say?" , "- blue; - assent" , "August" , "First-rate" , "- Icing, - flush, - jelly" .)