Quality of hawk or cuckoo, e.g., soaring across sierra (10)
I believe the answer is:
aggression
'quality of hawk' is the definition.
(hawk can mean someone promoting military action)
'cuckoo e g soaring across sierra' is the wordplay.
'cuckoo' is an anagram indicator (I've seen this in other clues).
'across' indicates putting letters inside.
'sierra' becomes 'S' (phonetic alphabet: alpha, bravo, charlie etc.).
'e'+'g'+'soaring'='egsoaring'
'egsoaring' anagrammed gives 'aggresion'.
'aggresion' going around 's' is 'AGGRESSION'.
'or' acts as a link.
(Other definitions for aggression that I've seen before include "Belligerence -hostility" , "Violent hostility" , "Use of armed force" , "warlike attitude" , "Unprovoked attack" .)