Deftness used if sense goes astray (7)
I believe the answer is:
finesse
'deftness' is the definition.
(thesaurus)
'used if sense goes astray' is the wordplay.
'used' indicates an anagram.
'goes astray' indicates an anagram.
'if' is an anagram of 'fi'.
'sense' is an anagram of 'nesse'.
'fi'+'nesse'='FINESSE'
(Other definitions for finesse that I've seen before include "Subtlety and tact" , "Sophistication" , "Skilful elegance or expertise" , "Refinement" , "isky card play" .)