Attacker losing a knight perhaps (5)
I believe the answer is:
rider
'knight perhaps' is the definition.
'rider' can be an answer for 'knight' (I've seen this before). I am not sure about the 'perhaps' bit.
'attacker losing a' is the wordplay.
'attacker' becomes 'raider' (I've seen this before).
'losing' suggests deleting specific letters.
'raider' with 'a' removed is 'RIDER'.
Can you help me to learn more?
(Other definitions for rider that I've seen before include "Person on horseback" , "Revere, for example" , "Condition added to a legal document" , "Jury's added comment" , "Supplement to verdict" .)