X on a wood projection (5)
I believe the answer is:
tenon
'a wood projection' is the definition.
'tenon' can be an answer for 'projection' (tenon is a kind of projection). I'm unsure of the 'a wood' bit.
'x on' is the wordplay.
'x' becomes 'ten' (X is the Roman numeral for ten).
'ten'+'on'='TENON'
Can you help me to learn more?
(Other definitions for tenon that I've seen before include "bit of wood?" , "Kind of saw" , "Tongue of wood, fits into mortise" , "Mortise filler" , "Part of joint that fits in mortise" .)