Part of wall fitted with extra breadth: that's hard to understand (6)
I believe the answer is:
gabble
'that's hard to understand' is the definition.
(gabble can mean unintelligible talk)
'part of wall fitted with extra' is the wordplay.
'part of wall' becomes 'gable' (I've seen this before).
'fitted with' means one lot of letters goes inside another.
'extra' becomes 'b' (abbreviation for 'bye' - cricket).
'gable' placed around 'b' is 'GABBLE'.
'breadth' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
(Other definitions for gabble that I've seen before include "Speak quickly and incessantly" , "Talk quickly and indistinctly" , "Speak rapidly and incessantly" , "Chatter inarticulately" , "Fast unintelligible talk" .)