Unpunctuality of feasts eaten by sailors (9)
I believe the answer is:
tardiness
'unpunctuality' is the definition.
(I've seen this before)
'feasts eaten by sailors' is the wordplay.
'feasts' becomes 'dines' (I've seen this in other clues).
'eaten by' is an insertion indicator (inserted letters get eaten).
'sailors' becomes 'tars' (tar is a kind of sailor).
'dines' going into 'tars' is 'TARDINESS'.
'of' acts as a link.
(Other definitions for tardiness that I've seen before include "Lateness" , "being a little late" , "Unpunctuality" , "No quality in fast food" , "Habit of being late" .)