Valve to raise, lest endless suffering should follow (11)
I believe the answer is:
cockleshell
'valve' is the definition.
(valve can mean part of a shell)
'raise lest endless suffering should follow' is the wordplay.
'raise' becomes 'cock' (eg a dog can cock his leg).
'endless' means to remove the last letter.
'suffering' becomes 'hell' (hell can mean a terrible ordeal).
'should follow' means one lot of letters go next to another (some letters go after others).
'lest' with its last letter removed is 'les'.
'cock'+'les'+'hell'='COCKLESHELL'
'to' acts as a link.
(Other definitions for cockleshell that I've seen before include "Very small boat (found in contrary Mary's garden?)" , "Light flimsy boat (poet.)" , "Protection for mollusc - flimsy boat" , "Small shallow boat" .)