Horseman's answer ignored by attacker (5)
I believe the answer is:
rider
'horseman's' is the definition.
(thesaurus)
'answer ignored by attacker' is the wordplay.
'answer' becomes 'a' (abbreviation eg in Q and A).
'ignored by' is a deletion indicator.
'attacker' becomes 'raider' (I've seen this before).
'raider' with 'a' removed is 'RIDER'.
(Other definitions for rider that I've seen before include "Equestrian or biker?" , "Condition added to a legal document" , "Contractual clause" , "One backing a horse" , "H. - Haggard, English novelist (She, etc)" .)