It's not the main change at rare packing company (5,5)
I believe the answer is:
terra firma
'it's not the main' is the definition.
I can't tell whether this defines the answer.
'change at rare packing company' is the wordplay.
'change' indicates an anagram.
'packing' is an insertion indicator (some letters are packed inside).
'company' becomes 'firm' (firm can mean a company or business).
'at'+'rare'='atrare'
'atrare' anagrammed gives 'terraa'.
'terraa' enclosing 'firm' is 'TERRA-FIRMA'.
Can you help me to learn more?
(Other definitions for terra firma that I've seen before include "Solid earth (Latin)" , "Hard ground" , "Solid earth - dry land" , "Solid ground" .)