Things improvised to restrict Post Office swindles (3-4)
I believe the answer is:
rip-offs
'things improvised' is the definition.
I can't judge whether this defines the answer.
'to restrict post office swindles' is the wordplay.
'to restrict' means one lot of letters goes inside another (inserted letters are restricted or contained).
'post office' becomes 'po' (abbreviation).
'swindles' becomes 'riffs' (I can't explain this - if you can you should give a lot more credence to this answer).
'po' put into 'riffs' is 'RIP-OFFS'.
Can you help me to learn more?
(Other definitions for rip-offs that I've seen before include "Financial exploitations" , "Informal swindles or frauds - by tearaways?" .)