A wood pile finally burning (5)
I believe the answer is:
afire
'burning' is the definition.
(thesaurus)
'a wood pile finally' is the wordplay.
'wood' becomes 'fir' (fir is a kind of wood).
'finally' suggests the final letters.
The final letter of 'pile' is 'e'.
'a'+'fir'+'e'='AFIRE'
(Other definitions for afire that I've seen before include "in flames" , "Burning" , "Ablaze" , "Alight" , "lit up" .)