Uncle Arthur not entirely lucid (5)
I believe the answer is:
clear
'lucid' is the definition.
('clear' can be similar in meaning to 'lucid')
'uncle arthur not entirely' is the wordplay.
'not entirely' indicates a hidden word.
'CLEAR' is hidden in the letters of 'uncle arthur'.
(Other definitions for clear that I've seen before include "Easily understood" , "Transparent or cloudless" , "Obvious; jump over" , "Bright" , "Understandable" .)