Stands tall when boarding ship (6)
I believe the answer is:
stalls
'stands' is the definition.
(eg stands or stalls at a market)
'tall when boarding ship' is the wordplay.
'when boarding' indicates putting letters inside.
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'tall' inserted within 'ss' is 'STALLS'.
(Other definitions for stalls that I've seen before include "Most expensive theatre seats" , "Compartments in stables or theatres" , "Seating on the ground floor of a theatre" , "Booths" , "Of engine, cuts out" .)