A salty thing, Boatman called 'saint with heart of stone' (4)
I believe the answer is:
miso
'a salty thing' is the definition.
(I know that miso is salty)
'boatman called saint with heart of stone' is the wordplay.
I cannot quite see how this works, but
'boatman' could be 'i' (where the crossword setter is 'Boatman') and 'i' is located in the answer.
'saint' could be 's' and 's' is present in the answer.
'of' could be 'o' and 'o' is present in the answer.
A single letter 'm' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for miso that I've seen before include "Soya-beam paste" , "Soya bean paste" , "Thick paste used in Japanese soups and sauces" .)