One choosing empty life in particular part of national activity (8)
I believe the answer is:
selector
'one choosing empty life in particular part of national activity' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'one choosing empty life in' is the wordplay.
'one choosing' becomes 'sector' (I can't explain this - if you can you should give a lot more credence to this answer).
'empty' says to hollow out the word (remove centre letters).
'in' is an insertion indicator.
'life' with its middle taken out is 'le'.
'sector' going around 'le' is 'SELECTOR'.
Can you help me to learn more?
(Other definitions for selector that I've seen before include "A chooser of electors" , "One who chooses" , "Person choosing (team)" , "Corselet (anag.)" .)