Jet plane initially in terrible rust (5)
I believe the answer is:
spurt
'jet' is the definition.
(I know that jet can be written as spurt)
'plane initially in terrible rust' is the wordplay.
'initially' suggests taking the first letters.
'in' is an insertion indicator.
'terrible' indicates anagramming the letters.
The first letter of 'plane' is 'p'.
'rust' is an anagram of 'surt'.
'p' put inside 'surt' is 'SPURT'.
(Other definitions for spurt that I've seen before include "Gush; sudden burst" , "Gush out suddenly" , "Short spell of increased speed" , "A short burst of activity" , "Jet, gush" .)