Shaft ending in channel secured by tool (4)
I believe the answer is:
axle
'shaft' is the definition.
(axle is a kind of shaft)
'ending in channel secured by tool' is the wordplay.
'ending in' suggests the final letters.
'secured by' is an insertion indicator (inserted letters are held or secured).
'tool' becomes 'axe' (axe is a kind of tool).
The last letter of 'channel' is 'l'.
'l' put inside 'axe' is 'AXLE'.
(Other definitions for axle that I've seen before include "Rotating spindle" , "Rod at centre of wheel" , "Wheel support" , "Wheel pivot" , "Wheel rotates around it" .)