The difference in currency does make things hard (7)
I believe the answer is:
stiffen
'make things hard' is the definition.
I can't judge whether this defines the answer.
'the difference in currency' is the wordplay.
'the difference' becomes 'tiff' (tiff is a kind of difference).
'in' means one lot of letters goes inside another.
'currency' becomes 'sen' (I can't justify this - if you can you should give a lot more credence to this answer).
'tiff' put within 'sen' is 'STIFFEN'.
'does' acts as a link.
Can you help me to learn more?
(Other definitions for stiffen that I've seen before include "Make unbending" , "Give strength to" , "Become more rigid" , "Reinforce" , "Make hard" .)