There's conflict when payment is not in sterling! (5)
I believe the answer is:
clash
'not in sterling' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'there's conflict when payment' is the wordplay.
'there's' means one lot of letters goes inside another (I've seen this in other clues).
'conflict when' becomes 'l' (I can't explain this - if you can you should believe this answer much more).
'payment' becomes 'cash' (cash is a kind of payment).
'l' going within 'cash' is 'CLASH'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for clash that I've seen before include "jar" , "Conflict, between colours say" , "Forceful disagreement" , "Punk group; disagree" , "Fail to fit" .)