Boils mineral aboard ship (5)
I believe the answer is:
sores
'boils' is the definition.
(I've seen this in another clue)
'mineral aboard ship' is the wordplay.
'mineral' becomes 'ore' (ore is a kind of mineral).
'aboard' is an insertion indicator.
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'ore' inserted within 'ss' is 'SORES'.
(Other definitions for sores that I've seen before include "Lesions" , "Skin infections" , "Painful spots on roses" , "Diseased spots" .)