Says they're in favour of using piece of wood before play begins in earnest (8)
I believe the answer is:
prologue
'earnest' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'says they're in favour of using piece of wood before play begins' is the wordplay.
I cannot really see how this works, but
'of' could be 'o' and 'o' is found within the answer.
'piece' could be 'r' (abbreviation for Rook) and 'r' is found in the answer.
'wood' could be 'log' (a log is a piece of wood) and 'log' is found within the answer.
'before' could be 'e' (I've seen this in other clues) and 'e' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'in' is the link.
Can you help me to learn more?
(Other definitions for prologue that I've seen before include "Introductory passage" , "Introductory section (of, eg, a play)" , "Start to play" , "Foreword" , "Dramatic preface" .)