In case of emergencies, cut piece of wood (4)
I believe the answer is:
spar
'piece of wood' is the definition.
'in case of emergencies cut' is the wordplay.
'in case of emergencies' becomes 'spare'.
'cut' means to remove the last letter.
'spare' with its final letter taken off is 'SPAR'.
(Other definitions for spar that I've seen before include "Box for practice" , "Pole to support sails, rigging etc" , "Box (gently) - fight (verbally)" , "Length of wood" , "Box gently; main beam of aircraft wing" .)