A lot of water? Enough at first in old container (5)
I believe the answer is:
ocean
'a lot' is the definition.
(I have seen 'lot of liquid' mean 'ocean' so perhaps 'lot' could also mean 'ocean')
'enough at first in old container' is the wordplay.
'at first' suggests taking the first letters.
'in' means one lot of letters goes inside another.
'old' becomes 'o' (abbreviation).
'container' becomes 'can' (can is a kind of container).
The first letter of 'enough' is 'e'.
'o'+'can'='ocan'
'e' placed within 'ocan' is 'OCEAN'.
'of water?' acts as a 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 ocean that I've seen before include "Big sea" , "Large stretch of sea" , "Large body of salt water" , "Large area of water" , "lot of liquid" .)