Ring doughnut at last comes in for my sweet (5)
I believe the answer is:
petal
'my sweet' is the definition.
(both are terms of endearment)
'ring doughnut at last comes in' is the wordplay.
'ring' becomes 'peal' (pealing is a kind of ringing).
'at last' indicates one should take the final letters.
'comes in' means one lot of letters goes inside another.
The final letter of 'doughnut' is 't'.
'peal' going around 't' is 'PETAL'.
'for' is the link.
(Other definitions for petal that I've seen before include "Plate (anag.)" , "Corolla leaf" , "Coloured segment of flower" , "part taken from stock?" , "Brightly-coloured part of flower" .)