Speak to tot on ship (7)
I believe the answer is:
address
'speak to' is the definition.
(synonyms)
'tot on ship' is the wordplay.
'tot' becomes 'add' (synonyms).
'on' becomes 're' ('on' can be mean 'with reference to').
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'add'+'re'+'ss'='ADDRESS'
(Other definitions for address that I've seen before include "Treat" , "Speak formally to an audience" , "Give a talk to" , "Think about" , "Where one lives" .)