Pool balls backsliding new girl pots (6)
I believe the answer is:
lagoon
'pool' is the definition.
(I've seen this before)
'balls backsliding new girl pots' is the wordplay.
'balls' becomes 'oo' (visually similar).
'backsliding' says the letters should be written in reverse.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'girl' becomes 'gal' ().
'pots' means one lot of letters goes inside another (pot can mean to contain).
'n'+'gal'='ngal'
'ngal' back-to-front is 'lagn'.
'oo' put within 'lagn' is 'LAGOON'.
(Other definitions for lagoon that I've seen before include "Body of water cut off from a larger body by a reef" , "Calm water behind reef" , "Salt water lake surrounded by reef or sand" , "Lake inside coral reef" , "Stretch of water inside coral reef or sandbank" .)