Trouble turning steel nut (8)
I believe the answer is:
unsettle
'trouble' is the definition.
(thesaurus)
'turning steel nut' is the wordplay.
'turning' is an anagram indicator.
'steel'+'nut'='steelnut'
'steelnut' is an anagram of 'UNSETTLE'.
(Other definitions for unsettle that I've seen before include "worry" , "Disturb, disconcert" , "Discompose" , "Disturb the calm or stability of" , "Disrupt" .)