Gather in the morning on a steamship (5)
I believe the answer is:
amass
'gather in' is the definition.
(I've seen this in another clue)
'morning on a steamship' is the wordplay.
'morning' becomes 'am' (as in 12:00 AM).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'steamship' becomes 'ss' (nautical abbreviation).
'am'+'a'+'ss'='AMASS'
'the' acts as a link.
(Other definitions for amass that I've seen before include "Stock up on" , "Sodium chloride" , "run up?" , "Build up" , "Get together" .)