Sail with mostly crude map at sea? This regatta’s had its day (8,3)

'regatta's had its day' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'sail with mostly crude map at sea?' is the wordplay.
'with' means one lot of letters go next to another.
'mostly' means to remove the last letter (most but not all of the word).
'at sea?' indicates anagramming the letters (I've seen 'at sea' mean this).
'crude' with its last letter removed is 'crud'.
'sail'+'crud'+'map'='sailcrudmap'
'sailcrudmap' anagrammed gives 'ADMIRALS CUP'.

'this' is the link.

Can you help me to learn more?