A mistake going flat out around the bend! (5)
I believe the answer is:
fault
'a mistake' is the definition.
(I know that mistake can be written as fault)
'flat out around the bend' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'around' indicates putting letters inside.
'the bend' becomes 'U' (a U-bend is a kind of bend - pipe under sink).
'flat' with letters rearranged gives 'falt'.
'falt' placed around 'u' is 'FAULT'.
'going' is the link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for fault that I've seen before include "Defect; blunder" , "Defect; culpability" , "Sin, error" , "Geological strata dislocation" , "Defect or imperfection" .)