Thick poles embedded in water (5)
I believe the answer is:
dense
'thick' is the definition.
(synonyms)
'poles embedded in water' is the wordplay.
'poles' becomes 'NS' (north and south poles).
'embedded in' means one lot of letters goes inside another (I've seen this in other clues).
'water' becomes 'dee' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ns' inserted within 'dee' is 'DENSE'.
Can you help me to learn more?
(Other definitions for dense that I've seen before include "Close-packed" , "Close - compact - stupid" , "Close-knit" , "Compressed - obtuse" , "Heavy" .)