Additional clause, article ignored by stock market speculator (5)
I believe the answer is:
rider
'additional clause' is the definition.
(additional clause on a contract or agreement)
'article ignored by stock market speculator' is the wordplay.
'article' becomes 'a' (the indefinite article in English).
'ignored by' is a deletion indicator.
'stock market speculator' becomes 'raider' (investor who targets undervalued companies).
'raider' with 'a' removed is 'RIDER'.
(Other definitions for rider that I've seen before include "Gold coin" , "Jury's recommendation" , "Condition - under which a horse races" , "Person on horse" , "Condition added to a legal document" .)