Day that is long (3)
I believe the answer is:
die
'long' is the definition.
(to long for or to die for something is to crave it)
'day that is' is the wordplay.
'day' becomes 'd' (abbreviation for day).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'd'+'ie'='DIE'
(Other definitions for die that I've seen before include "Fail" , "One of a pair of dice" , "Pass away" , "Stop functioning" , "Cease to exist" .)