Horde of people, upset about defeat, to become more mature (7)
I believe the answer is:
blossom
'become more mature' is the definition.
(blossom can mean to mature or develop)
'horde of people upset about defeat' is the wordplay.
'horde of people' becomes 'mob' (horde can mean a mob or crowd of people).
'upset' is a reversal indicator.
'about' is an insertion indicator.
'defeat' becomes 'loss' (I've seen this before).
'mob' backwards is 'bom'.
'bom' going around 'loss' is 'BLOSSOM'.
'to' acts as a link.
(Other definitions for blossom that I've seen before include "Mass of flowers - mature" , "Flowers on a tree/bush" , "Flowers on tree or bush" , "Produce flowers" , "Flowers on, eg, cherry" .)