Precipitation during plane excursion initially results in headache (8)
I believe the answer is:
migraine
'headache' is the definition.
(migraine is a kind of headache)
'precipitation during plane excursion initially' is the wordplay.
'precipitation' becomes 'rain' (rain is a kind of precipitation).
'during' indicates putting letters inside.
'plane' becomes 'mig' (fighter jet produced by Russian company MiG).
'initially' indicates taking the first letters.
The initial letter of 'excursion' is 'e'.
'mig'+'e'='mige'
'rain' put inside 'mige' is 'MIGRAINE'.
'results in' acts as a link.
(Other definitions for migraine that I've seen before include "Sick headache" , "Recurrent throbbing headache" , "Long nerve fibre" , "terrible pain" , "From mare gin I get a severe headache" .)