One of those involved in lock-in is a key figure? (6)
I believe the answer is:
jailer
'figure?' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for jailer that I've seen before include "Custodian of a prisoner" , "Old-fashioned screw" , "Turnkey" , "Sort of guard" , "Prison warder" .)