Aircraft for Black Sabbath initially (4)
I believe the answer is:
jets
'aircraft' is the definition.
(jet is a type of aircraft)
'black sabbath initially' is the wordplay.
'black' becomes 'jet' (jet black is a kind of black).
'initially' suggests taking the first letters.
The initial letter of 'sabbath' is 's'.
'jet'+'s'='JETS'
'for' is the link.
(Other definitions for jets that I've seen before include "Rapid streams of liquid or gas" , "Aeroplanes - Sharks rivals in "West Side Story"" , "Water spouts or fast planes" , "Fast airplanes" .)