Substitute is excited, on edge in volatile semi (9)
I believe the answer is:
euphemism
'substitute' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'excited on edge in volatile semi' is the wordplay.
'excited on' becomes 'up' (I am not sure about the 'on' bit.).
'edge' becomes 'hem' (I've seen this before).
'in' means one lot of letters goes inside another.
'volatile' indicates an anagram.
'up'+'hem'='uphem'
'semi' with letters rearranged gives 'eism'.
'uphem' going within 'eism' is 'EUPHEMISM'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for euphemism that I've seen before include "Milder substitute word" , "Milder term" , "Inoffensive expression substituted for one that may be offensive" , "Replacement for a term of abuse?" , "Inoffensive term used instead of an unpleasant one" .)