She has to live on a ship (4)
I believe the answer is:
bess
'she' is the definition.
'live on a ship' is the wordplay.
'live' becomes 'be' (synonyms).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'a ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'be'+'ss'='BESS'
'has to' acts as a link.
(Other definitions for bess that I've seen before include "Black - was Dick Turpin's horse" , "Nickname for English queen" , "Elizabeth (dim.)" , "Operatic mate for Porgy" , "Our Good Queen" .)