Valve in vessels raised angle (8)
I believe the answer is:
stopcock
'valve' is the definition.
(I've seen this before)
'vessels raised angle' is the wordplay.
'vessels' becomes 'pots' (pot is a kind of vessel).
'raised' says the letters should be written backwards (going up in a down clue).
'angle' becomes 'cock' (as in a dog cocking his leg).
'pots' in reverse letter order is 'stop'.
'stop'+'cock'='STOPCOCK'
'in' is the link.
(Other definitions for stopcock that I've seen before include "Water controlled by this" , "Halt rooster with pipe valve" , "Water/gas supply valve" , "Desist, male bird! it's a valve in a pipe" , "Isolating valve in pipework" .)