Get used to upset stomach, mainly after ingesting copper (8)
I believe the answer is:
accustom
'get used' is the definition.
(I've seen this before)
'upset stomach mainly after ingesting copper' is the wordplay.
'upset' indicates an anagram.
'mainly' means to remove the last letter (virtually or mainly the word, but not all of it).
'after ingesting' indicates putting letters inside.
'copper' becomes 'Cu' (chemical symbol).
'stomach' with its last letter taken off is 'stomac'.
'stomac' anagrammed gives 'acstom'.
'acstom' going around 'cu' is 'ACCUSTOM'.
'to' acts as a link.
(Other definitions for accustom that I've seen before include "Make (someone) used to (something)" , "Maybe reconcile" , "Familiarise" , "get one inured to" , "Adapt" .)