Ben blocks busy bus lane - shocking! (11)
I believe the answer is:
unspeakable
'shocking' is the definition.
(thesaurus)
'ben blocks busy bus lane' is the wordplay.
'ben' becomes 'peak' ('ben' is a Scottish term for peak, as in Ben Nevis).
'blocks' means one lot of letters goes inside another (upper walls off glass inside formal).
'busy' indicates anagramming the letters.
'bus'+'lane'='buslane'
'buslane' anagrammed gives 'unsable'.
'peak' put within 'unsable' is 'UNSPEAKABLE'.
(Other definitions for unspeakable that I've seen before include "Is impossible pronunciation so" , "The hardest tongue twisters are" , "Too bad for words" , "Was Marcel Marceau's act so" , "dreadful" .)