Fifty-one pound fluid (6)
I believe the answer is:
liquid
'fluid' is the definition.
(I know that fluid can be written as liquid)
'fifty-one pound' is the wordplay.
'fifty' becomes 'li' (I've seen this in other clues).
'one pound' becomes 'quid' (UK slang for a pound).
'li'+'quid'='LIQUID'
(Other definitions for liquid that I've seen before include "Spendable" , "Such an easy source of funds" , "Neither gas nor solid" , "Sloppy" , "Neither sold nor gaseous" .)