European spies held by China grow weak (8)
I believe the answer is:
emaciate
'grow weak' is the definition.
('emaciate' can be a synonym of 'grow weak')
'european spies held by china' is the wordplay.
'european' becomes 'e' (abbreviation e.g. EU).
'spies' becomes 'cia'.
'held by' means one lot of letters goes inside another.
'china' becomes 'mate' ('China plate' is rhyming slang for 'mate').
'cia' going inside 'mate' is 'maciate'.
'e'+'maciate'='EMACIATE'
(Other definitions for emaciate that I've seen before include "Make extremely thin" , "Waste away" , "get very thin" .)