A transgression in Egypt's capital is very silly (7)
I believe the answer is:
asinine
'very silly' is the definition.
(I've seen this before)
'a transgression in egypt's capital' is the wordplay.
'transgression' becomes 'sin' (sin is a kind of transgression).
'capital' says to take the initial letters.
The initial letter of 'egypts' is 'e'.
'a'+'sin'+'in'+'e'='ASININE'
'is' acts as a link.
(Other definitions for asinine that I've seen before include "Absurd" , "Stupid (as a donkey?)" , "In the nature of supposedly stupid animal" , "Really silly, ridiculous" , "Idiotic" .)