Wager about right length of planking (6)
I believe the answer is:
strake
'length of planking' is the definition.
(length of planking on a ship)
'wager about right' is the wordplay.
'wager' becomes 'stake' (wager in poker).
'about' is an insertion indicator.
'right' becomes 'r' (abbreviation).
'stake' placed around 'r' is 'STRAKE'.
(Other definitions for strake that I've seen before include "Odd takers of a ship's plank" , "Line of planking in the hull of a wooden ship" , "Section of wheel rim; line of plating (along ship)" , "Hull section" , "Aerodynamic ridge" .)