Black steamship carries old chief (4)
I believe the answer is:
boss
'chief' is the definition.
(synonyms)
'black steamship carries old' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'steamship' becomes 'ss' (nautical abbreviation).
'carries' says to put letters next to each other (in a down clue, some letters go beneath others).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'ss' after 'o' is 'oss'.
'b'+'oss'='BOSS'
(Other definitions for boss that I've seen before include "Behave overbearingly" , "Governor" , "Manager (colloq.); stud" , "Master - stud" , "Bully" .)