Hard to stop prey flying after unknown wind (6)
I believe the answer is:
zephyr
'wind' is the definition.
(I know that zephyr is a type of wind)
'hard to stop prey flying after unknown' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'to stop' is an insertion indicator (stop can mean to plug a hole in something).
'flying' indicates anagramming the letters (letters fly around into new positions).
'after' says to put letters next to each other.
'unknown' becomes 'z' (algebra).
'prey' is an anagram of 'epyr'.
'h' placed inside 'epyr' is 'ephyr'.
'ephyr' after 'z' is 'ZEPHYR'.
(Other definitions for zephyr that I've seen before include "Slight breeze" , "A soft gentle breeze" , "Light wind" , "A gentle 16" , "A soft gentle wind" .)