Prevent bird reaching valve (8)
I believe the answer is:
stopcock
'valve' is the definition.
(type of valve)
'prevent bird' is the wordplay.
'prevent' becomes 'stop' (stopping is a kind of preventing).
'bird' becomes 'cock' (cock is a kind of bird).
'stop'+'cock'='STOPCOCK'
'reaching' is the link.
(Other definitions for stopcock that I've seen before include "Short pipe opened by turning a handle" , "Water controlled by this" , "it may interrupt the flow" , "water regulator" , "Regulator of flow" .)