A valve to move slowly if absolutely necessary (2,1,5)
I believe the answer is:
at a pinch
'if absolutely necessary' is the definition.
(I've seen this before)
'a valve to move slowly' is the wordplay.
'valve' becomes 'tap'.
'to move slowly' becomes 'inch' (I've seen this before).
'a'+'tap'+'inch'='AT A PINCH'
(Other definitions for at a pinch that I've seen before include "in a crisis?" , "if need be" , "In case of necessity" , "If really necessary" , "In time of difficulty or necessity - nip it" .)