Thin straight bar (3)
I believe the answer is:
rod
'thin straight bar' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for rod that I've seen before include "Instrument of punishment" , "Baton, pole" , "Angler's cane" , "Little boy" , "Spare it and spoil the child, they used to say" .)