Army's initial breaches endanger sovereign (8)
I believe the answer is:
imperial
'sovereign' is the definition.
(I've seen this before)
'army's initial breaches endanger' is the wordplay.
'initial' suggests taking the first letters.
'breaches' is an insertion indicator (some letters breach another word).
'endanger' becomes 'imperil' (synonyms).
The initial letter of 'armys' is 'a'.
'a' put within 'imperil' is 'IMPERIAL'.
(Other definitions for imperial that I've seen before include "Of UK weights and measures" , "Sovereign - beard - size of paper" , "Non-metric" , "Magnificent; small pointed beard" , "Sovereign - old Russian gold coin - type of beard" .)