Left almost anything to crack up (5)
I believe the answer is:
laugh
'crack up' is the definition.
(I've seen this in another clue)
'left almost anything' is the wordplay.
'left' becomes 'l' (common abbreviation).
'almost' means to remove the last letter (most of the word but not all of it).
'anything' becomes 'aught' (I've seen this before).
'aught' with its final letter removed is 'augh'.
'l'+'augh'='LAUGH'
'to' is the link.
(Other definitions for laugh that I've seen before include "Break up" , "Good fun" , "Cachinnate" , "scream?" , "Fun person" .)