Leader of the first army (4)
I believe the answer is:
star
'leader' is the definition.
(thesaurus)
'of the first army' is the wordplay.
'of' indicates a hidden word.
'STAR' is hidden in the letters of 'first army'.
(Other definitions for star that I've seen before include "Heavenly body - leading actress?" , "Rush" , "Leading actor in a show" , "Leading character" , "Play a leading role" .)