Let this be a warning to you - coffee seed is firm inside (6)
I believe the answer is:
beacon
'let this be a warning to you coffee' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'seed is firm inside' is the wordplay.
'seed' becomes 'bean' (bean is a kind of seed**).
'is firm' becomes 'co' (a firm is a company).
'inside' is an insertion indicator.
'bean' placed around 'co' is 'BEACON'.
Can you help me to learn more?
(Other definitions for beacon that I've seen before include "Signal-fire" , "Navigational guide" , "Signal fire - lighthouse" , "Warning fire or signal" , "Guiding light?" .)