Comedian is fool somewhat lacking: tolerable cracks (5)
I believe the answer is:
joker
'comedian' is the definition.
(I've seen this before)
'fool somewhat lacking tolerable cracks' is the wordplay.
'fool' becomes 'jerk' (I've seen this before).
'somewhat lacking' means to remove the last letter (most but not all of the word).
'tolerable' becomes 'ok'.
'cracks' indicates putting letters inside (some letters create an opening in the word).
'jerk' with its final letter taken off is 'jer'.
'jer' placed around 'ok' is 'JOKER'.
'is' acts as a link.
(Other definitions for joker that I've seen before include "Prankster" , "Pack's extra card" , "Card - wag" , "Humorist" , "Comedian" .)