Thursday elevenses — small, not large — including a whole lot of water (3,5,4)
I believe the answer is:
the seven seas
'whole lot of water' is the definition.
I know nothing about this answer so I cannot tell whether this works.
'thursday elevenses small not large including a' is the wordplay.
I cannot really understand how this works, but
'thursday' could be 'th' (common abbreviation) and 'th' is located in the answer.
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is present in the answer.
'including' could be 'even' ('even' can mean 'including' when reciting a list) and 'even' is present in the answer.
'a' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for the seven seas that I've seen before include "All the world's oceans in general" , "Arctic etc" .)