Dredge unfinished part of swimming pool (6)
I believe the answer is:
deepen
'dredge' is the definition.
(I've seen this in another clue)
'unfinished part of swimming pool' is the wordplay.
'unfinished' means to remove the last letter.
'part of swimming pool' becomes 'deep end' (I've seen this before).
'deepend' with its final letter taken away is 'DEEPEN'.
(Other definitions for deepen that I've seen before include "Make or become more profound" , "Get stronger" , "Intensify" , "make room for more water" , "Increase in depth" .)