Character rapidly going round the bend (5)
I believe the answer is:
faust
'character' is the definition.
(I know that Faust is a character)
'rapidly going round the bend' is the wordplay.
'rapidly' becomes 'fast' (synonyms).
'going round' is an insertion indicator (some letters go around others).
'the bend' becomes 'U' (a U-bend is a kind of bend - pipe under sink).
'fast' enclosing 'u' is 'FAUST'.
(Other definitions for faust that I've seen before include "Alchemist" , "He sold his soul to the Devil in exchange for knowledge" , "Tragic figure" , "Goethe play" , "1 28 [EPIC POEM]" .)