Analyses for purity when eating second perhaps (6)
I believe the answer is:
assays
'analyses for purity' is the definition.
(to assay is to test the quality of a metal)
'when eating second perhaps' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'eating' means one lot of letters goes inside another.
'second' becomes 's' (common abbreviation).
'perhaps' becomes 'say' (both can mean 'for example').
's'+'say'='ssay'
'as' placed around 'ssay' is 'ASSAYS'.
(Other definitions for assays that I've seen before include "Tests ore for quality" , "Tests ore for mineral content" , "Analyses (chemical substances)" , "Tests to determine metallic content" , "Determines the proporation of gold in a nugget" .)