Curb something of rare interest (4)
I believe the answer is:
rein
'curb' is the definition.
(I've seen this in another clue)
'something of rare interest' is the wordplay.
'something of' indicates the answer is hidden within the clue (I've seen 'something' mean this (I've seen this in other clues)).
'REIN' is hidden in the letters of 'rare interest'.
(Other definitions for rein that I've seen before include "kidney?" , "one's attached to a bit" , "given free to experienced riders?" , "Restraining lead" , "Controlling strap" .)