Plunderer overlooking a person on horseback (5)
I believe the answer is:
rider
'person on horseback' is the definition.
(someone riding a horse)
'plunderer overlooking a' is the wordplay.
'plunderer' becomes 'raider' (synonyms).
'overlooking' suggests deleting specific letters.
'raider' with 'a' removed is 'RIDER'.
(Other definitions for rider that I've seen before include "H. - Haggard, English novelist (She, etc)" , "recommendation for mercy?" , "Traveller on horseback" , "one making journey" , "Legal document's additional clause" .)