Slovakia's imprisoning number two spy (5)
I believe the answer is:
spook
'spy' is the definition.
(spook can informally mean a spy)
'slovakia's imprisoning number two' is the wordplay.
'slovakia' becomes 'SK' (abbreviation for Slovakia).
'imprisoning' is an insertion indicator.
'number two' becomes 'poo' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'sk' enclosing 'poo' is 'SPOOK'.
Can you help me to learn more?
(Other definitions for spook that I've seen before include "Enterprise propels him" , "Ghost, spectre" , "agent" , "Frighten - haunt - spy" , "Ghost - US spy" .)