Sleep on small wooden planks (7)
I believe the answer is:
slumber
'sleep' is the definition.
(I know that sleep can be written as slumber)
'small wooden planks' is the wordplay.
I cannot really see how this works, but
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is present in the answer.
The remaining letters 'lumber' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'on' acts as a link.
Can you help me to learn more?
(Other definitions for slumber that I've seen before include "Doze" , "Overnight social event for (American) teenage girls" , "snores, maybe?" , "Rumbles (anag.)" , "repose" .)