Crowd comes back having swallowed defeat — that's mature (7)
I believe the answer is:
blossom
'mature' is the definition.
(I've seen this before)
'crowd comes back having swallowed defeat' is the wordplay.
'crowd' becomes 'mob' (mob can mean a crowd of people).
'comes back' says the letters should be written backwards.
'having swallowed' indicates putting letters inside.
'defeat' becomes 'loss' (I've seen this before).
'mob' back-to-front is 'bom'.
'bom' placed around 'loss' is 'BLOSSOM'.
'that's' is the link.
(Other definitions for blossom that I've seen before include "Produce flowers" , "Flowers on a tree/bush" , "Bloom" , "Mass of flowers - mature" , "May, possibly" .)