Military forces lose many in the stars (5)
I believe the answer is:
aries
'stars' is the definition.
(I've seen this before)
'military forces lose many' is the wordplay.
'military forces' becomes 'armies' (I've seen this in another clue).
'lose' is a deletion indicator.
'many' becomes 'm' (Roman numeral for a thousand).
'armies' with 'm' taken out is 'ARIES'.
'in the' is the link.
(Other definitions for aries that I've seen before include "star group" , "A sign from Heaven" , "Philip -, English poet" , "Zodiacal sign also known as the Ram" , "Constellation with sign of the ram" .)