Crazy house internally a big let-down (6)
I believe the answer is:
bathos
'a big let-down' is the definition.
'crazy house internally' is the wordplay.
'crazy' becomes 'bats' (I've seen this before).
'house' becomes 'ho' (abbreviation for house).
'internally' indicates putting letters inside.
'bats' placed around 'ho' is 'BATHOS'.
(Other definitions for bathos that I've seen before include "Serious-to-trivial switch" , "Trite anticlimax" , "Ludicrous descent of mood in writing" , "Unintentional lowering of tone" , "Change from serious to trivial (in literature)" .)