Trained in secret as underground opposition (10)
I believe the answer is:
resistance
'underground opposition' is the definition.
'resistance' can be an answer for 'opposition' (synonyms). I am unsure of the 'underground' bit.
'trained in secret as' is the wordplay.
'trained' indicates anagramming the letters (to train can mean to grow something into a shape).
'in'+'secret'+'as'='insecretas'
'insecretas' with letters rearranged gives 'RESISTANCE'.
Can you help me to learn more?
(Other definitions for resistance that I've seen before include "Organised covert opposition to an occupying power" , "Rebellious group" , "defiance" , "underground movement" , "Rebels" .)