An excess of fluid in the tissues (6)
I believe the answer is:
oedema
'an excess of fluid in the tissues' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for oedema that I've seen before include "it gets bigger" , "Excessive fluid in tissues causing swelling" , "Condition of excess fluid in the tissues" , "Excess of fluids in the tissues" , "Dropsy" .)