Unfairly implies one is lacking in, or devoid of intelligence (6)
I believe the answer is:
simple
'devoid of intelligence' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'unfairly implies one is lacking in' is the wordplay.
'unfairly' indicates anagramming the letters (I've seen this in other clues).
'one' becomes 'i' (Roman numeral).
'is lacking in' indicates named letters should be taken away.
'implies' with 'i' taken out is 'mplies'.
'mplies' with letters rearranged gives 'SIMPLE'.
'or' is the link.
Can you help me to learn more?
(Other definitions for simple that I've seen before include "Plant with medicinal properties" , "Elementary" , "Easy or uncomplicated" , "Easy; plain" , "half-witted" .)