Skin condition initially contracted by sailor that is on board (7)
I believe the answer is:
scabies
'skin condition' is the definition.
(medical complaint)
'initially contracted by sailor that is on board' is the wordplay.
'initially contracted by sailor' becomes 'cab' (I can't justify this - if you can you should give a lot more credence to this answer).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'on' is an insertion indicator.
'board' becomes 'ss' (I've seen this in other clues).
'cab'+'ie'='cabie'
'cabie' going within 'ss' is 'SCABIES'.
Can you help me to learn more?
(Other definitions for scabies that I've seen before include "Contagious skin disease causing itching" , "rash complaint?" , "Contagious skin infection" , "Skin disease - "The Itch"" , "Contagious skin disease that causes itching" .)