Perhaps once held a lot of water (5)
I believe the answer is:
ocean
'lot of water' is the definition.
'ocean' can be an answer for 'water' (ocean is a kind of water). I am not sure about the 'lot of' bit.
'perhaps once held a' is the wordplay.
'perhaps' indicates anagramming the letters.
'held' indicates putting letters inside.
'once' is an anagram of 'ocen'.
'ocen' enclosing 'a' is 'OCEAN'.
Can you help me to learn more?
(Other definitions for ocean that I've seen before include "Indian or Arctic, say" , "eg Indian, Atlantic" , "Huge body of water" , "Vast sea" , "One of the Seven Seas" .)