A measure to restrict strike if absolutely necessary (2,1,5)
I believe the answer is:
at a pinch
'if absolutely necessary' is the definition.
(at a pinch means if required)
'a measure to restrict strike' is the wordplay.
'measure' becomes 'inch' (inch is a unit of length).
'to restrict' means one lot of letters goes inside another (inserted letters are restricted or trapped).
'strike' becomes 'tap' (tap can mean to strike or hit).
'a'+'inch'='ainch'
'ainch' going around 'tap' is 'AT A PINCH'.
(Other definitions for at a pinch that I've seen before include "in a crisis?" , "if need be" , "under pressure" , "In case of necessity" , "in case of emergency" .)