Peacekeeper with the German pup one is likely to lose (8)
I believe the answer is:
underdog
'likely to lose' is the definition.
'peacekeeper with the german pup one' is the wordplay.
'peacekeeper' becomes 'un' (I've seen this in other clues).
'with' is a charade indicator (letters next to each other).
'the german' becomes 'der' ('the' in German).
'pup one' becomes 'dog' (I am not sure about this - if you are sure you should believe this answer much more).
'un'+'der'+'dog'='UNDERDOG'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for underdog that I've seen before include "unlikely winner" , "One with little status" , "Likely loser" , "One of low status" , "Lower-rated competitor" .)