It’s just beginning to sell in markets (5,4)
I believe the answer is:
fairs fair
'markets' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'it's just beginning to sell in' is the wordplay.
'it's' becomes 'fair' (I can't justify this - if you can you should give a lot more credence to this answer).
'just' becomes 'fair' (just can mean fair in the sense of showing justice).
'beginning to' says to take the initial letters.
'in' means one lot of letters goes inside another.
The first letter of 'sell' is 's'.
'fair'+'fair'='fairfair'
'fairfair' going around 's' is 'FAIRS FAIR'.
Can you help me to learn more?
(Other definitions for fairs fair that I've seen before include "play the game" , "Appeal for just treatment" .)