Old sailor preferred dealing with explosive ingredient (9)
I believe the answer is:
saltpetre
'explosive ingredient' is the definition.
(I've seen this before)
'old sailor preferred dealing with' is the wordplay.
'old sailor' becomes 'salt' (old-fashioned nickname for a sailor).
'preferred' becomes 'pet' ('pet' can be a synonym of 'preferred').
'dealing with' becomes 're' (regarding).
'salt'+'pet'+'re'='SALTPETRE'
(Other definitions for saltpetre that I've seen before include "chemical" , "Explosive ingredient" , "It's used as a fertilizer and explosive" , "Gunpowder ingredient" , "Compound" .)