If absolutely necessary
'if absolutely necessary' is the definition.
(at a pinch means if required)
This is all the clue.
'if absolutely necessary' is the definition.
(at a pinch means if required)
This is the entire clue.
(Other definitions for at a pinch that I've seen before include "If really necessary" , "in case of emergency" , "if need be" , "in a crisis?" , "In case of necessity" .)