Move cautiously after a short 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)
'move cautiously after a short strike' is the wordplay.
'move cautiously' becomes 'inch' (I've seen this in another clue).
'after' means one lot of letters go next to another.
'short strike' becomes 'tap' (a tap is a slight hit).
'a'+'tap'='atap'
'inch' after 'atap' is 'AT A PINCH'.
(Other definitions for at a pinch that I've seen before include "In case of necessity" , "If really necessary" , "In time of difficulty or necessity - nip it" , "in case of emergency" , "in a crisis?" .)