Low-flying extra-terrestrial flier (5)
I believe the answer is:
'flier' is the definition.
(owlet is a kind of birdanimal)
'low-flying extra-terrestrial' is the wordplay.
'flying' indicates anagramming the letters.
'extra terrestrial' becomes 'et' (abbreviation - like in the famous 1982 movie).
'low' with letters rearranged gives 'owl'.
(Other definitions for owlet that I've seen before include "Young nocturnal bird of prey" , "new hooter?" , "Offspring of the wise one?" , "Small bird of prey from Africa and Asia" , "Baby bird of prey" .)