Do this in advance, before it's 80% drained (7)
I believe the answer is:
preempt
'80% drained' is the definition.
I can't tell whether this defines the answer.
'do this in advance before it's' is the wordplay.
'do' becomes 'emp' (I can't explain this - if you can you should believe this answer much more).
'this in advance' becomes 'pre' (prefix meaning 'before').
'before' means one lot of letters go next to another.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'emp' after 'pre' is 'preemp'.
'preemp'+'t'='PREEMPT'
Can you help me to learn more?
(Another definition for preempt that I've seen is " Appropriate".)