Go with a girl, the ultimate in charm (3)
I believe the answer is:
vim
'go' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'with a girl the ultimate in charm' is the wordplay.
'with' means one lot of letters go next to another.
'a girl' becomes 'Vi' (short for Violet).
'the ultimate in' indicates one should take the final letters (take the ultimate ie last letter).
The final letter of 'charm' is 'm'.
'vi' put next to 'm' is 'VIM'.
Can you help me to learn more?
(Other definitions for vim that I've seen before include "Energy and vigour" , "Liveliness" , "Informal energy or drive" , "Pizazz" , "Pep (informal)" .)