Oxygen a half-cut relative needed in watering-hole (5)
I believe the answer is:
oasis
'watering-hole' is the definition.
(I've seen this before)
'oxygen a half-cut relative' is the wordplay.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'half cut' suggests halving the letters.
'relative' becomes 'sister' (sister is a kind of relative).
'sister' cut in half is 'sis'.
'o'+'a'+'sis'='OASIS'
'needed in' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for oasis that I've seen before include "place of relief" , "Florist's foam" , "Fertile tract in a desert" , "Pop group" , "Fertile place in a desert, a watering hole" .)