May upset writer about the first remittance (7)
I believe the answer is:
payment
'remittance' is the definition.
(I've seen this in another clue)
'may upset writer about the first' is the wordplay.
'upset' is an anagram indicator.
'writer' becomes 'pen' (something which writes).
'about' means one lot of letters goes inside another.
'first' indicates taking the first letters.
The first letter of 'the' is 't'.
'may' with letters rearranged gives 'aym'.
'aym' put within 'pen' is 'paymen'.
'paymen'+'t'='PAYMENT'
(Other definitions for payment that I've seen before include "Sum of money handed over" , "Discharge of an obligation with money" , "Settlement, remittance" , "Handing over money" , "Reward for work" .)