Cat in the past goes round the bend (5)
I believe the answer is:
ounce
'cat' is the definition.
(ounce is a kind of cat)
'past goes round the bend' is the wordplay.
'past' becomes 'once' ('once' can be similar in meaning to 'past').
'goes round' indicates putting letters inside.
'the bend' becomes 'u' (a U-bend is a kind of bend - pipe under sink).
'once' enclosing 'u' is 'OUNCE'.
'in the' is the link.
(Other definitions for ounce that I've seen before include "scrap" , "Lightweight" , "Fraction of a pound" , "Wild cat - small weight" , "On cue (anag.)" .)