A number advanced, going into a joint (5)
I believe the answer is:
tenon
'a joint' is the definition.
(I've seen this before)
'a number advanced going' is the wordplay.
'a number' becomes 'ten' (ten is a number).
'advanced going' becomes 'on' (both can mean functioning or activated. I am not sure about the 'advanced' bit.).
'ten'+'on'='TENON'
'into' acts as a link.
Can you help me to learn more?
(Other definitions for tenon that I've seen before include "Mortise extension" , "One projecting" , "Outstanding" , "piece of carpentry?" , "Wooden projection shaped to fit in a mortise joint" .)