Foolish son entering excellent square (7)
I believe the answer is:
asinine
'foolish' is the definition.
('asinine' can be similar in meaning to 'foolish')
'son entering excellent square' is the wordplay.
'son' becomes 's'.
'entering' means one lot of letters goes inside another.
'excellent' becomes 'ai'.
'square' becomes 'nine' (I've seen this before**).
's' inserted within 'ai' is 'asi'.
'asi'+'nine'='ASININE'
(Other definitions for asinine that I've seen before include "Jenny's often like this" , "Stupid (as a donkey?)" , "Idiotic" , "Senseless" , "Daft" .)