Military commander is horrified, losing good man (4)
I believe the answer is:
agha
'military commander' is the definition.
(historical title in the Ottoman Empire)
'horrified losing good man' is the wordplay.
'horrified' becomes 'aghast' (I've seen this before).
'losing' suggests deleting specific letters.
'good man' becomes 'st'.
'aghast' with 'st' taken out is 'AGHA'.
'is' is the link.
(Other definitions for agha that I've seen before include "Turkish civil or military leader" , "Alternative spelling for a Turkish commander" , "Turkish officer" , "Ruler" .)