Rod's large cutting tool (4)
I believe the answer is:
axle
'rod's' is the definition.
(axle is a kind of rod)
'large cutting tool' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'cutting' means one lot of letters goes inside another (some letters cut their way into a word).
'tool' becomes 'axe' (axe is a kind of tool).
'l' placed into 'axe' is 'AXLE'.
(Other definitions for axle that I've seen before include "Spindle between wheels" , "Rod at centre of wheel" , "Mandrel" , "It's pivotal" , "Rotating shaft" .)