Respond to a trace of difference (5)
I believe the answer is:
react
'respond' is the definition.
(I know that respond can be written as react)
'trace of difference' is the wordplay.
'of difference' indicates an anagram (I've seen 'difference' mean this).
'trace' is an anagram of 'REACT'.
'to a' acts as a link.
(Other definitions for react that I've seen before include "Hit back" , "Take" , "Respond to something" , "Perform again" , "Retaliate" .)