He is one not lacking bit of nous, but simple (10)
I believe the answer is:
elementary
'simple' is the definition.
(synonyms)
'he is one not lacking bit of nous' is the wordplay.
I cannot quite see how this works, but
'he' could be 'lem' and 'lem' is located in the answer.
'one' could be 'a' and 'a' is located in the answer.
'not' could be 'nt' (synonyms) and 'nt' is found within the answer.
The remaining letters 'eery' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'but' acts as a link.
Can you help me to learn more?
(Other definitions for elementary that I've seen before include "Like some particles" , "One precursor to 7 [SECONDARY MODERN]" , "schooling?" , "Simple, my dear Watson!" , "ABC!" .)