Not great pool of correspondence (4)
I believe the answer is:
pond
'not great pool' is the definition.
'pond' can be an answer for 'pool' ('pond' can be a synonym of 'pool'). I am unsure of the 'not great' bit.
'of correspondence' is the wordplay.
'of' indicates the answer is hidden within the clue.
'POND' can be found hidden inside 'correspondence'.
Can you help me to learn more?
(Other definitions for pond that I've seen before include "Small body of water" , "Pool of still water" , "Small lake" , "Small area of still water" , "Himalayan country" .)