Crowd, upset about defeat, burst out (7)
I believe the answer is:
blossom
'burst out' is the definition.
(blossom can mean to burst out or emerge)
'crowd upset about defeat' is the wordplay.
I cannot quite understand how this works, but
'crowd' could be 'm' and 'm' is present in the answer.
'defeat' could be 'loss' (I've seen this before) and 'loss' is found within the answer.
The remaining letters 'bo' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for blossom that I've seen before include "Do well" , "Flowers on, eg, cherry" , "Produce flowers" , "Mass of flowers - mature" , "Fruit tree flowers" .)