Marauder, having lost face, is up in arms (5)
I believe the answer is:
irate
'marauder having lost' is the definition.
I can't judge whether this definition defines the answer.
'face is up in arms' is the wordplay.
'face' indicates taking the first letters (I've seen 'face of' mean this).
'up in arms' becomes 'rate' (I can't justify this - if you can you should give a lot more credence to this answer).
The first letter of 'is' is 'i'.
'i'+'rate'='IRATE'
Can you help me to learn more?
(Other definitions for irate that I've seen before include "Magical rod" , "Ticked off" , "Full of 22" , "Narked" , "Livid" .)