A desperate man embraces a complicated, ghastly theme (5,7)
I believe the answer is:
danse macabre
'ghastly theme' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'a desperate man embraces a complicated' is the wordplay.
'a desperate man' becomes 'dan' (I am not sure about the 'desperate' bit.).
'complicated' indicates anagramming the letters (a complex or confusing ordering of the letters).
'embraces'+'a'='embracesa'
'embracesa' anagrammed gives 'semacabre'.
'dan'+'semacabre'='DANSE MACABRE'
Can you help me to learn more?
(Other definitions for danse macabre that I've seen before include "moves leading to grave" , "Gory scene" , "Work of Saint-Saens; dance of death" .)