Part of pool table ultimately breaks bank (4,3)
I believe the answer is:
deep end
'part of pool' is the definition.
(area in a swimming pool)
'table ultimately breaks bank' is the wordplay.
'ultimately' says to take the final letters.
'breaks' means one lot of letters goes inside another.
'bank' becomes 'depend' (I've seen this before).
The last letter of 'table' is 'e'.
'e' put inside 'depend' is 'DEEP END'.
(Other definitions for deep end that I've seen before include "Part of pool for diving" , "difficult spot" , "could be drowning not waving here" , "Place for diving board" , "Where to dive in a pool" .)