Admitting defeat, gang returns to develop well (7)
I believe the answer is:
blossom
'develop well' is the definition.
(to blossom is to develop or flourish)
'admitting defeat gang returns' is the wordplay.
'admitting' indicates putting letters inside.
'defeat' becomes 'loss' (I've seen this before).
'gang' becomes 'mob' (synonyms).
'returns' shows that the letters should be reversed in order.
'mob' back-to-front is 'bom'.
'loss' inserted into 'bom' is 'BLOSSOM'.
'to' is the link.
(Other definitions for blossom that I've seen before include "Do well" , "Come into flower" , "Fruit tree flowers" , "Bloom" , "Mass of flowers - mature" .)