Shark fighters in Jaws: film replacing a western (4)
I believe the answer is:
jets
'shark fighters' is the definition.
(the Jets and Sharks are enemies in West Side Story)
'jaws film replacing a western' is the wordplay.
I cannot quite understand how this works, but
'film' could be 'et' (E.T. the Extra-Terrestrial) and 'et' is found in the answer.
This explanation may well be incorrect...
'in' acts as a link.
(Other definitions for jets that I've seen before include "Aeroplanes - Sharks rivals in "West Side Story"" , "Water spouts or fast planes" , "Fast airplanes" , "Rapid streams of liquid or gas" .)