When there's a lack of water (6)
I believe the answer is:
nowell
'when there's a' is the definition.
I can't judge whether this definition defines the answer.
'lack of water' is the wordplay.
'lack of' becomes 'no' (eg no bread means a lack of bread).
'water' becomes 'well' (well water is a kind of water).
'no'+'well'='NOWELL'
Can you help me to learn more?
(Other definitions for nowell that I've seen before include "The First - - (carol)" , "Carol refrain" , "Old name for Christmas" , "Christmas (in carols)" .)