It's highly desirable to send back amount, that, in short, vexes first pair (4-4)
I believe the answer is:
must-have
'in short vexes first pair' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'it's highly desirable to send back amount' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'highly desirable' becomes 'have' (I can't justify this - if you can you should give a lot more credence to this answer).
'to' says to put letters next to each other (I've seen this in other clues).
'send back' shows that the letters should be reversed in order.
'amount' becomes 'sum' (synonyms).
'sum' backwards is 'mus'.
't'+'have'='thave'
'thave' after 'mus' is 'MUST-HAVE'.
'that' acts as a link.
Can you help me to learn more?
(Other definitions for must-have that I've seen before include "Essential" , "Highly desirable item" .)