Upturned tub, tip-top for holding water (4)
I believe the answer is:
butt
'holding water' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'upturned tub tip-top' is the wordplay.
'upturned' says the letters should be written in reverse.
'tip-top' becomes 't' (top of the word 'tip-').
'tub' back-to-front is 'but'.
'but'+'t'='BUTT'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for butt that I've seen before include "Person who is a target for ridicule" , "Target < vessel" , "Scapegoat - strike - bottom - barrel" , "ram > end" , "Fag-end" .)