Headache jet fighter mainly precipitated (8)
I believe the answer is:
migraine
'headache' is the definition.
(migraine is a kind of headache)
'jet fighter mainly precipitated' is the wordplay.
'jet fighter' becomes 'mig' (I've seen this in other clues).
'mainly' means to remove the last letter.
'precipitated' becomes 'rained' (raining is a kind of precipitating).
'rained' with its last letter taken off is 'raine'.
'mig'+'raine'='MIGRAINE'
(Other definitions for migraine that I've seen before include "Severe recurring headache" , "It brings severe headache" , "From mare gin I get a severe headache" , "terrible pain" , "Severe recurrent headache" .)