Black rock hits small flies (4)
I believe the answer is:
jets
'flies' is the definition.
(jetting is a kind of flying)
'black rock hits small' is the wordplay.
'black rock' becomes 'jet' (jet is a black stone).
'hits' says to put letters next to each other (some letters hit or touch others).
'small' becomes 's' (abbreviation - e.g. clothes size).
'jet'+'s'='JETS'
(Other definitions for jets that I've seen before include "Fast airplanes" , "Aeroplanes - Sharks rivals in "West Side Story"" , "Rapid streams of liquid or gas" , "Water spouts or fast planes" .)