Flyer’s downfall, circling oil installation ahead of time (7,4)
I believe the answer is:
frigate bird
'flyer's' is the definition.
(something which flies)
'downfall circling oil installation ahead of time' is the wordplay.
'downfall' becomes 'fate' (I can't justify this - if you can you should give a lot more credence to this answer).
'circling' means one lot of letters goes inside another (circling can mean going around or containing).
'oil installation' becomes 'rig' (as in an oil rig).
'ahead of' is a charade indicator (letters next to each other) (I've seen this in other clues).
'time' becomes 'bird' (both are slang for a prison sentence).
'fate' going around 'rig' is 'frigate'.
'frigate'+'bird'='FRIGATE BIRD'
(Other definitions for frigate bird that I've seen before include "marine flyer" , "Tropical avian" , "I may be seen flying" .)