Unfathomable part in Shakespearean comedy? Not so (10)
I believe the answer is:
bottomless
'unfathomable' is the definition.
(similar in meaning)
'part in shakespearean comedy? not so' is the wordplay.
'part in shakespearean comedy?' becomes 'Bottom' (character in A Midsummer Night's Dream).
'not so' becomes 'less' (I've seen this before).
'bottom'+'less'='BOTTOMLESS'
(Other definitions for bottomless that I've seen before include "Inexhaustible" , "Very deep" , "without 17 [BEHINDS] !" , "Unfathomable" , "never getting used up?" .)