Difficult offer (6)
I believe the answer is:
killer
'offer' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'difficult' is the wordplay.
I cannot quite see how this works, but
'difficult' could be 'ill' (ill could historically mean difficult) and 'ill' is located in the answer.
The remaining letters 'ker' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for killer that I've seen before include "Slaughterer" , "Hilarious joke" , "great joke?" , "criminal?" , "executioner called for?" .)