Half-decent golden egg laid by crossword setters is not to be sniffed at (7)
I believe the answer is:
odorous
'sniffed at' is the definition.
The answer and definition are not the same part of speech. However, past participle verbs and adjectives sometimes define each other.
'half-decent golden egg laid by crossword setters is not' is the wordplay.
'half decent golden egg' becomes 'odor' (I can't explain this - if you can you should give a lot more credence to this answer).
'laid by' says to put letters next to each other.
'crossword setters' becomes 'us' (the crossword setter and their colleagues).
'is not' becomes 'o' (I've seen this in other clues).
'us' put after 'o' is 'ous'.
'odor'+'ous'='ODOROUS'
'to be' acts as a link.
Can you help me to learn more?
(Other definitions for odorous that I've seen before include "Sweet-smelling" , "Aromatic" , "Having a scent" , "Scented" , "having a distinct aroma" .)