He's a dud, taking in what may be all right (7)
I believe the answer is:
dullard
'he's a' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'dud taking in what may be all right' is the wordplay.
'taking in' indicates putting letters inside.
'what may be' indicates an anagram (I've seen 'what might be' mean this).
'right' becomes 'r' (common abbreviation).
'all'+'r'='allr'
'allr' with letters rearranged gives 'llar'.
'dud' placed around 'llar' is 'DULLARD'.
Can you help me to learn more?
(Other definitions for dullard that I've seen before include "A mentally slow person" , "idiot" , "Someone not very bright" , "Boring type" , "Numbskull" .)