Implied agreement a citation enshrines (5)
I believe the answer is:
tacit
'implied' is the definition.
(synonyms)
'agreement a citation enshrines' is the wordplay.
'enshrines' indicates the answer is hidden within the clue (the answer is enshrined or held within the clue).
'TACIT' can be found hidden inside 'agreement a citation'.
(Other definitions for tacit that I've seen before include "Aircraft engine casing" , "Attic? (anag)" , "Silent - understood" , "Implicitly understood" , "Implied but not directly expressed" .)