Sign an auditor’s letter (3)
I believe the answer is:
cue
'sign' is the definition.
(a cue is a sign or signal)
'an auditor's letter' is the wordplay.
'an auditor's' shows a homophone (sound like).
'letter' becomes 'q'.
'q' is a homophone of 'CUE'.
(Other definitions for cue that I've seen before include "Catchword" , "hit ball, in a way" , "Signal to players" , "Hint on stage" , "Snooker player's stick" .)