Among a number, approve memento (5)
I believe the answer is:
token
'memento' is the definition.
(I've seen this in another clue)
'among a number approve' is the wordplay.
'among' means one lot of letters goes inside another (inserted letters are placed among others).
'a number' becomes 'ten' (ten is a number).
'approve' becomes 'ok' (synonyms).
'ten' placed around 'ok' is 'TOKEN'.
(Other definitions for token that I've seen before include "Emblem" , "Keepsake or memorial as of friendship" , "Voucher (for, eg, books)" , "Sign - voucher - chip" , "Nominal; voucher" .)