River ploy upset a fellow on the up (9)
I believe the answer is:
resurgent
'up' is the definition.
Both the definition and answer are adjectives. Perhaps they are linked in a way I don't understand?
'river ploy upset a fellow on' is the wordplay.
I cannot quite see how this works, but
'river' could be 'r' (abbreviation) and 'r' is present in the answer.
'fellow' could be 'gent' (synonyms) and 'gent' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'the' acts as a link.
Can you help me to learn more?
(Other definitions for resurgent that I've seen before include "given new life" , "Reviving after period of inactivity" , "Reviving in popularity" , "Reviving after a lull" , "rising again" .)