Publicly expose a lightweight understudy (8)
I believe the answer is:
denounce
'understudy' is the definition.
Although both the answer and definition are verbs in their base form, I cannot see how one could define the other.
'publicly expose a lightweight' is the wordplay.
'publicly expose' becomes 'den' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a lightweight' becomes 'ounce' (I've seen this before).
'den'+'ounce'='DENOUNCE'
Can you help me to learn more?
(Other definitions for denounce that I've seen before include "Speak violently against" , "Revile" , "As threat, identify" , "Speak publicly against, censure" , "Inform against (someone)" .)