Close relative, one boarding steamship (3)
I believe the answer is:
sis
'close relative' is the definition.
(short for sister)
'one boarding steamship' is the wordplay.
'one' becomes 'i' (Roman numeral).
'boarding' is an insertion indicator.
'steamship' becomes 'ss' (nautical abbreviation).
'i' going inside 'ss' is 'SIS'.
(Other definitions for sis that I've seen before include "Secret Intelligence Service (abbr.)" , "Family member (informal)" , "Relative's" , "Secret Intelligence Service (abbreviation)" , "Sibling (informal)" .)