They fly low, perhaps, over the sierra (4)
I believe the answer is:
owls
'they fly' is the definition.
Although both the answer and 'fly' are nouns, they are not in the same form of the noun.
'low perhaps over the sierra' is the wordplay.
'perhaps' is an anagram indicator.
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'the sierra' becomes 'S' (phonetic alphabet: alpha, bravo, charlie etc.).
'low' anagrammed gives 'owl'.
'owl'+'s'='OWLS'
Can you help me to learn more?
(Other definitions for owls that I've seen before include "Slow kind of birds" , "Nocturnal bird" , "Strigiform creatures" , "Types incredibly wise" , "Nocturnal hooters?" .)