Cut a tool in half! (4)
I believe the answer is:
sawn
'cut' is the definition.
(cut with a saw)
'a tool in half' is the wordplay.
'a tool' becomes 'saw' (saw is a kind of tool).
'half' suggests halving the letters.
'in' halved is 'n'.
'saw'+'n'='SAWN'
(Other definitions for sawn that I've seen before include "went up and down?" , "timber must be" , "(Of wood) cut" , "Cut with serrated blade" , "Severed" .)