Red Rum cut through (4)
I believe the answer is:
ruby
'red' is the definition.
(I know that red can be written as ruby)
'rum cut through' is the wordplay.
'cut' means to remove the last letter.
'through' becomes 'by' ().
'rum' with its final letter removed is 'ru'.
'ru'+'by'='RUBY'
(Other definitions for ruby that I've seen before include "Stony girl" , "Deep red variety of corundum" , "killed an assassin" , "Gemstone - shade of red" , "Girl's name - colour" .)