Aha! Writer stocks trendy shop jars (3,5,5)
I believe the answer is:
the penny drops
'stocks trendy shop jars' is the definition.
I don't know anything about this answer so I can't tell whether this works.
'aha writer' is the wordplay.
I cannot really see how this works, but
'writer' could be 'pen' (something which writes) and 'pen' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for the penny drops that I've seen before include "Understanding finally dawns" , "Now I see it" , "Those sweets very cheap" , "Money falls" , "I understand" .)