Sandbar a grave by itself, first of ships being lost (7)

'sandbar' is the definition.
Both the answer and definition are singular nouns.
Perhaps there's a link between them I don't understand?

'a grave by itself first of ships being lost' is the wordplay.
'a grave' becomes 'tomb' (synonyms).
'by itself' shows that letters should be duplicated (the word appears by another version of itself).
'first of' becomes 'o' (1st letter of 'of').
'ships' means one lot of letters goes inside another (to ship can mean to carry or transport something).
'being lost' becomes 'l' (abbreviation in sporting league tables).
'o' doubled is 'oo'.
'oo' enclosing 'l' is 'olo'.
'tomb'+'olo'='TOMBOLO'

Can you help me to learn more?