Auditor’s function is to make an endorsement (4)
I believe the answer is:
sign
'make an endorsement' is the definition.
'sign' can be an answer for 'make' (thesaurus). I'm unsure of the 'an endorsement' bit.
'auditor's function' is the wordplay.
'auditor's' shows a homophone (sound like).
'function' becomes 'sine' (mathematical function).
'sine' sounds like 'SIGN'.
'is to' acts as a link.
Can you help me to learn more?
(Other definitions for sign that I've seen before include "Omen or indication" , "part of the zodiac" , "Pointer - indicate agreement in writing" , "Any of the 12 divisions of the Zodiac" , "Evidence" .)