The fellow isn't illiterate and dim (5)
I believe the answer is:
faint
'dim' is the definition.
(I know that dim can be written as faint)
'the fellow isn't' is the wordplay.
'the fellow' becomes 'f' (abbreviation - of a society etc.).
'isn't' becomes 'aint' (non-standard version of 'isn't').
'f'+'aint'='FAINT'
'illiterate and' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for faint that I've seen before include "Dim -- swoon" , "Swoon, lose consciousness" , "Indistinct - pass out" , "(Of a chance) slight" , "Weak" .)