Fake preparation starts with a couple of eggs; it’s for washing (7)
I believe the answer is:
shampoo
'for washing' is the definition.
(I've seen this before)
'fake preparation starts with a couple of eggs' is the wordplay.
'a couple of eggs' can mean two replacements for 'egg'.
'fake' becomes 'sham' (sham can mean fake or pretend).
'starts with' indicates taking the first letters (I've seen 'start with' mean this).
'egg' becomes 'o' (an 'o' resembles an egg).
'egg' becomes 'o' (an 'o' resembles an egg).
The first letter of 'preparation' is 'p'.
'sham'+'p'+'o'+'o'='SHAMPOO'
'it's' acts as a link.
(Other definitions for shampoo that I've seen before include "Hair-care product" , "cleansing liquid" , "Liquid soap" , "Hair-cleaner" , "Cleaner for locks" .)