Not allowed to reverse round the bend (3)
I believe the answer is:
out
'not allowed' is the definition.
(I've seen this before)
'to reverse round the bend' is the wordplay.
'reverse' shows that the letters should be reversed in order.
'round' indicates putting letters inside.
'the bend' becomes 'U' (a U-bend is a kind of bend - pipe under sink).
'to' backwards is 'ot'.
'ot' going around 'u' is 'OUT'.
(Other definitions for out that I've seen before include "Possibly caught" , "On strike - dismissed - unconsciousout" , "Inaccurate - dismissed" , "In error" , "sleeping" .)