This could cause rustiness in old worker, say, after eleven days (7)
I believe the answer is:
oxidant
'this could cause rustiness' is the definition.
The definition suggests a singular noun which matches the answer.
'old worker say after eleven days' is the wordplay.
'old' becomes 'o' (abbreviation).
'worker say' becomes 'ant' (worker is a type of ant).
'after' is a charade indicator (letters next to each other).
'eleven' becomes 'xi' (Roman numerals).
'days' becomes 'd' (abbreviation for days).
'xi'+'d'='xid'
'ant' after 'xid' is 'xidant'.
'o'+'xidant'='OXIDANT'
'in' is the link.
Can you help me to learn more?
(Other definitions for oxidant that I've seen before include "chemical agent" , "it produces rust" , "Something creating rust" , "Substance such as hydrogen peroxide" , "agent causing deterioration" .)