A little fluid, say, not a condition full of it (6)
I believe the answer is:
dropsy
'condition full of it' is the definition.
I can't tell whether this definition defines the answer.
'a little fluid say not a' is the wordplay.
'a little fluid' becomes 'drop' (a drop of liquid).
'not' is a deletion indicator.
'say' with 'a' taken out is 'sy'.
'drop'+'sy'='DROPSY'
(Other definitions for dropsy that I've seen before include "medical condition" , "Oedema (old name)" , "Accumulation of fluid in a part of the body - oedema" , "illness" , "fluid build-up" .)