It's usually clear when a girl comes to the wrong conclusion (5)
I believe the answer is:
glass
'it's usually clear' is the definition.
I can't judge whether this definition defines the answer.
'a girl comes to the wrong conclusion' is the wordplay.
'a girl' becomes 'lass' (lass can informally mean a girl).
'comes to' means one lot of letters go next to another.
'conclusion' says to take the final letters.
The final letter of 'wrong' is 'g'.
'lass' after 'g' is 'GLASS'.
'when' acts as a link.
Can you help me to learn more?
(Other definitions for glass that I've seen before include "measure of wine" , "that one can see through" , "Drink container" , "Translucent, usually transparent, material" , "Perhaps schooner" .)