Attacker not a motorcyclist? (5)
I believe the answer is:
rider
'motorcyclist?' is the definition.
(I've seen this before)
'attacker not a' is the wordplay.
'attacker' becomes 'raider' (I've seen this before).
'not' is a deletion indicator.
'raider' with 'a' removed is 'RIDER'.
(Other definitions for rider that I've seen before include "given little weight" , "Clause appended to a legislative bill" , "knight" , "Equestrian added by a jury?" , "Condition added to a legislative bill" .)