Auditor's folk investigating source of firewood? (5)
I believe the answer is:
copse
'source of firewood?' is the definition.
(a copse is a small wood)
'auditor's folk investigating' is the wordplay.
'auditor's' shows a homophone (sound like).
'folk investigating' becomes 'cops' (police officers).
'cops' sounds like 'COPSE'.
(Other definitions for copse that I've seen before include "Harvested woodland" , "Stand of trees" , "Coppice" , "Small group of trees" , "Dense thicket of trees" .)