Timber not turning — it's drifting in the water (8)
I believe the answer is:
plankton
'drifting in the water' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'timber not turning' is the wordplay.
'timber' becomes 'plank' (plank is a kind of timber).
'turning' shows that the letters should be reversed in order.
'not' back-to-front is 'ton'.
'plank'+'ton'='PLANKTON'
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for plankton that I've seen before include "food for school, possibly" , "drifting masses" , "Simple marine life" , "Whale's staple diet" , "Food source for aquatic creatures" .)