Firms gathered zinc occasionally that changes appearance (8)
I believe the answer is:
cosmetic
'changes appearance' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'firms gathered zinc occasionally' is the wordplay.
'firms' becomes 'cos' (co is short for company - a business or firm).
'gathered' becomes 'met' ('meet' can be a synonym of 'gather').
'occasionally' means one should take alternating letters.
The alternate letters of 'zinc' are 'ic'.
'cos'+'met'+'ic'='COSMETIC'
'that' is the link.
Can you help me to learn more?
(Other definitions for cosmetic that I've seen before include "Slap on the face" , "Superficial - lipstick, say" , "Preparation designed to beautify the body" , "Beauty preparation" , "improving superficial appearance" .)