Flow of excess water never blocking wading bird (3-3)
I believe the answer is:
run-off
'flow' is the definition.
(I know this)
'never blocking wading bird' is the wordplay.
'never' becomes 'no' (I've seen this in other clues).
'blocking' is an insertion indicator.
'wading bird' becomes 'ruff' (ruff is a kind of wading bird).
'no' put within 'ruff' is 'RUN-OFF'.
'of excess water' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for run-off that I've seen before include "Further contest to decide a tie" , "Draining of rainfall from an area" .)