Marked keys in wrong date (6)
I believe the answer is:
tagged
'marked' is the definition.
('tag' can be a synonym of 'mark')
'keys in wrong date' is the wordplay.
'keys' becomes 'gg' (two G's - as in the musical key of G).
'in' is an insertion indicator.
'wrong' is an anagram indicator.
'date' is an anagram of 'taed'.
'gg' inserted within 'taed' is 'TAGGED'.
(Other definitions for tagged that I've seen before include "Labelled (electronically?)" , "Trailed along - labelled" , "What follower did" , "Touched a player while s/he was holding the ball" , "Identified electronically" .)