Tool cuts a cow's tail (7)
I believe the answer is:
hacksaw
'tool' is the definition.
(hacksaw is a kind of tool)
'cuts a cow's tail' is the wordplay.
'cuts' becomes 'hacks' (hacking is a kind of cutting**).
'tail' suggests the final letters.
The last letter of 'cow' is 'w'.
'hacks'+'a'+'w'='HACKSAW'
(Other definitions for hacksaw that I've seen before include "means of cutting lead article?" , "something to cut copper?" , "something cutting" , "metalworker may use?" , "cutting edge" .)