Spots yours truly wearing leather on board ship (6)
I believe the answer is:
stains
'spots' is the definition.
(spot can mean a stain or mark)
'yours truly wearing leather on board ship' is the wordplay.
'yours truly' becomes 'I' (the person who set this crossword).
'wearing' means one lot of letters goes inside another.
'leather' becomes 'tan' (tan can mean to convert into leather).
'on board' is an insertion indicator (inserted letters are taken on board).
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'i' going within 'tan' is 'tain'.
'tain' placed inside 'ss' is 'STAINS'.
(Other definitions for stains that I've seen before include "Discolorations on satins" , "Discolours or dyes wood" , "Discolorations or blemishes" , "Dirty marks" , "Discolorations or dirt marks" .)