Evidence of flight building up to arrival (6,5)
I believe the answer is:
vapour trail
'evidence of flight' is the definition.
The definition suggests a singular noun which matches the answer.
'building up to arrival' is the wordplay.
'building' indicates anagramming the letters.
'up'+'to'+'arrival'='uptoarrival'
'uptoarrival' anagrammed gives 'VAPOUR TRAIL'.
Can you help me to learn more?
(Other definitions for vapour trail that I've seen before include "Visible line left in the sky by an aircraft" , "Line of cloud following an aeroplane" , "Track of aircraft in the sky" .)