Soar past broken-down thing, unreliable (3-2-5)
I believe the answer is:
fly-by-night
'unreliable' is the definition.
(I've seen this before)
'soar past broken-down thing' is the wordplay.
'soar past' becomes 'flyby' (I can't justify this - if you can you should give a lot more credence to this answer).
'broken down' is an anagram indicator.
'thing' is an anagram of 'night'.
'flyby'+'night'='FLY-BY-NIGHT'
Can you help me to learn more?
(Other definitions for fly-by-night that I've seen before include "Unreliable - person who departs secretly to avoid debts" , "Cowboy" , "Unreliable, untrustworthy" , "Transitory" , "Unreliable, shady" .)