Somehow drain mere to reveal what's left (9)
I believe the answer is:
remainder
'what's left' is the definition.
'somehow drain mere' is the wordplay.
'somehow' is an anagram indicator.
'drain'+'mere'='drainmere'
'drainmere' with letters rearranged gives 'REMAINDER'.
'to reveal' is the link.
(Other definitions for remainder that I've seen before include "Rest - sell off (books) cheap" , "Leftovers" , "Unsold book; rest" , "Sell off book stock cheaply" , "Dispose of unsold copies of book at a reduced price" .)