Excellent ability to discern military standard watch (5,3)
I believe the answer is:
eagle eye
'watch' is the definition.
(thesaurus)
'excellent ability to discern military standard' is the wordplay.
I cannot really understand how this works, but
'ability' could be 'eye' (eye is a kind of ability) and 'eye' is found in the answer.
'standard' could be 'eagle' (ancient Roman standard or symbol) and 'eagle' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for eagle eye that I've seen before include "Keenness of vision" , "Critical inspection" , "(Person with) very keen vision" , "one envisages a long way off" , "Ability to observe fine detail" .)