Rattle steel nut loose (8)
I believe the answer is:
unsettle
'rattle' is the definition.
(I know that unsettle is a more specific form of rattle)
'steel nut loose' is the wordplay.
'loose' indicates an anagram (letters are loosened from their original positions).
'steel'+'nut'='steelnut'
'steelnut' with letters rearranged gives 'UNSETTLE'.
(Other definitions for unsettle that I've seen before include "Nettle us so as to disturb" , "Disturb, disconcert" , "Fluster" , "Make uneasy, disturb the calm of" , "Trouble" .)