Followed secretly (6)
I believe the answer is:
tailed
'followed secretly' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tailed that I've seen before include "Followed, tracked" , "Dogged" , "Followed by private detective, having rear appendage" , "Followed (suspect)" , "Followed informally in detail" .)