Fine blue stream (4)
I believe the answer is:
flow
'stream' is the definition.
('flow' can be a synonym of 'stream')
'fine blue' is the wordplay.
'fine' becomes 'f' (abbreviation).
'blue' becomes 'low' (I've seen this in other clues).
'f'+'low'='FLOW'
(Other definitions for flow that I've seen before include "Move along as a stream" , "Steady continuous stream" , "Move steadily" , "Run or move like a liquid" , "Fluid motion" .)