Gets wise as British coinage declines (3,5,5)
I believe the answer is:
the penny drops
'gets wise' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'british coinage declines' is the wordplay.
'british' becomes 'the' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'coinage' becomes 'penny' (penny is a kind of coinage).
'declines' becomes 'drops' (dropping is a kind of declining).
'the'+'penny'+'drops'='THE PENNY DROPS'
'as' acts as a link.
Can you help me to learn more?
(Other definitions for the penny drops that I've seen before include "Understanding finally dawns" , "Money falls" , "I comprehend" , "what happens in slot machine?" , "Now I see it" .)