Silly girl that is going out (5)
I believe the answer is:
inane
'silly' is the definition.
(I've seen this in another clue)
'girl that is going out' is the wordplay.
I cannot quite see how this works, but
'girl' could be 'ina' and 'ina' is present in the answer.
The remaining letters 'ne' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for inane that I've seen before include "Without meaning or point" , "Mindlessly silly" , "Fatuous, vacuous" , "Moronic" , "Idiotic" .)