Light reversed in fleeting image (6)
I believe the answer is:
ignite
'light' is the definition.
(I know that ignite is the is a more specific form of the action of light.)
'reversed in fleeting image' is the wordplay.
'reversed' is a reversal indicator.
'in' indicates a hidden word.
'etingi' can be found hidden inside 'fleeting image'.
'etingi' written backwards gives 'IGNITE'.
(Other definitions for ignite that I've seen before include "'Set alight, fire up (6)'" , "Catch fire; provoke" , "Swan constellation" , "Spark off" , "Cause to catch fire" .)