Fish that is lacking tail (4)
I believe the answer is:
ides
'fish' is the definition.
(ide is a type of fish)
'that is lacking tail' is the wordplay.
'that is' becomes 'id est' ('that is' in Latin).
'lacking tail' means to remove the last letter.
'idest' with its final letter removed is 'IDES'.
(Other definitions for ides that I've seen before include "March day fatal to Julius Caesar" , "Mid-month date in Roman calendar" , "Julius Caesar died in these March days" , "March day Caesar died" , "Caesar was killed in this side of March" .)