Score well under par of the Roman military standard (5)
I believe the answer is:
eagle
This could be a double definition.
'score well under par' is the first definition.
'eagle' can be an answer for 'score' (eagling is a kind of scoring). I'm unsure of the 'well under par' bit.
'roman military standard' is the second definition.
'eagle' can be an answer for 'standard' (ancient Roman standard or symbol). I am not certain of the 'roman military' bit.
'of the' acts as a link.
Can you help me to learn more?
(Other definitions for eagle that I've seen before include "born hunter" , "Erne, perhaps" , "Bird of prey; former children's comic" , "Avian symbol of power" , "five buck bit, or more" .)