Where one is out of action
I believe the answer is:
acton
'where' is the definition.
The definition and answer are not the same part of speech.
'one is out of action' is the wordplay.
'one' becomes 'i' (Roman numeral).
'is out of' suggests deleting specific letters.
'action' with 'i' removed is 'ACTON'.
Can you help me to learn more?
(Other definitions for acton that I've seen before include "somewhere in London" , "Respond in accordance with - London district" , "District of West London" , "Part of the London borough of Ealing - British historian, d. 1902" , "London area" .)