Jack prepared mainly orange seasoning (8)
I believe the answer is:
tarragon
'seasoning' is the definition.
(tarragon is a kind of seasoning)
'jack prepared mainly orange' is the wordplay.
'jack' becomes 'tar' (both are informal terms for sailor).
'prepared' indicates an anagram.
'mainly' means to remove the last letter.
'orange' with its final letter taken off is 'orang'.
'orang' anagrammed gives 'ragon'.
'tar'+'ragon'='TARRAGON'
(Other definitions for tarragon that I've seen before include "Strangely arrogant herb" , "Herb coming" , "(Aromatic plant producing) herb" , "Arrogant (anag) - herb" , "Culinary herb" .)