Unfathomable? A Midsummer Night’s Dream isn’t (10)
I believe the answer is:
bottomless
'unfathomable?' is the definition.
('bottomless' can be similar in meaning to 'unfathomable')
I can't explain the remainder of the clue.
(Other definitions for bottomless that I've seen before include "Inexhaustible" , "Having no limits" , "Very deep" , "never getting used up?" , "without 17 [BEHINDS] !" .)