A companion among number to get Mexican snack (5)
I believe the answer is:
nacho
'mexican snack' is the definition.
(I've seen this before)
'a companion among number' is the wordplay.
'companion' becomes 'ch' (Companion of Honour).
'among' indicates putting letters inside (inserted letters are placed among others).
'number' becomes 'no' (abbreviation for 'number').
'a'+'ch'='ach'
'ach' put into 'no' is 'NACHO'.
'to get' acts as a link.
(Other definitions for nacho that I've seen before include "Mexican food item" , "kind of chip" , "Tortilla chip served with savoury toppings" , "part of a snack?" , "Mexican corn chip eaten with cheese, etc" .)