Salad ingredient is copper brown when bit of celery added (8)
I believe the answer is:
cucumber
'salad ingredient' is the definition.
(I've seen this before)
'copper brown when bit of celery added' is the wordplay.
'copper' becomes 'Cu' (Cu is the chemical symbol for copper).
'brown when bit of celery' becomes 'cumber' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'added' says to put letters next to each other.
'cu'+'cumber'='CUCUMBER'
'is' is the link.
Can you help me to learn more?
(Other definitions for cucumber that I've seen before include "Oblong fruit used in salad" , "Cylindrical green fruit" , "Long salad vegetable" , "Green salad fruit" , "Ecu crumb (anag.)" .)