Imprison China's correspondents (3,4)
I believe the answer is:
pen pals
'correspondents' is the definition.
(pen pal is a kind of correspondent)
'imprison china's' is the wordplay.
'imprison' becomes 'pen' (to pen in is to imprison).
'china's' becomes 'pal's' (china plate is Cockney rhyming slang for mate).
'pen'+'pals'='PEN PALS'
(Other definitions for pen pals that I've seen before include "They correspond" , "Friends who communicate in writing" , "Young correspondents" , "'Friends through correspondence (3,4)'" .)