Unspoken agreement a citizen conceals (5)
I believe the answer is:
tacit
'unspoken' is the definition.
(I've seen this before)
'agreement a citizen conceals' is the wordplay.
'conceals' indicates a hidden word.
'TACIT' can be found hidden inside 'agreement a citizen'.
(Other definitions for tacit that I've seen before include "Wordless" , "Not said though understood" , "Implicit, unspoken" , "Implied, not spoken" , "Unspoken but understood" .)