Two axes and no going back? This is hard (4)
I believe the answer is:
onyx
'this is hard' is the definition.
The definition suggests a singular noun which matches the answer.
'two axes and no going back?' is the wordplay.
'two axes' becomes 'xy' (x axis and y axis. I am not sure about the 'two' bit.).
'and' is a charade indicator (letters next to each other).
'going back?' says the letters should be written backwards.
'xy'+'no'='xyno'
'xyno' back-to-front is 'ONYX'.
Can you help me to learn more?
(Other definitions for onyx that I've seen before include "Quartz used as a gem" , "Marbled stone with layers of colours" , "Kind of agate" , "Stone-like marble with coloured layers" , "Striped agate" .)