A dish of mealies formerly useful to navigators (3,4)
I believe the answer is:
sea mile
'a dish' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'mealies formerly useful to navigators' is the wordplay.
'mealies' anagrams to 'SEA MILE'.
However, I'm unsure how the anagram is indicated.
'of' is the link.
Can you help me to learn more?
(Other definitions for sea mile that I've seen before include "unit of distance" , "Nautical distance" .)