Mealies affected by nautical measure (3,4)
I believe the answer is:
sea mile
'nautical measure' is the definition.
(I've seen this in another clue)
'mealies affected' is the wordplay.
'affected' indicates an anagram.
'mealies' with letters rearranged gives 'SEA-MILE'.
'by' acts as a link.
(Other definitions for sea mile that I've seen before include "unit of distance" , "Nautical distance" .)