Junior to follow weaker party (8)
I believe the answer is:
underdog
'weaker party' is the definition.
'junior to follow' is the wordplay.
'junior' becomes 'under' (I've seen this in other clues).
'to follow' becomes 'dog' (I've seen this before).
'under'+'dog'='UNDERDOG'
(Other definitions for underdog that I've seen before include "One of low status" , "certainly not a favourite!" , "Likely loser" , "Canine expected to lose?" , "unlikely winner" .)