Trial for individual taken in by secret plan (5)
I believe the answer is:
pilot
'trial' is the definition.
(I've seen this before)
'individual taken in by secret plan' is the wordplay.
'individual' becomes 'i'.
'taken in by' means one lot of letters goes inside another.
'secret plan' becomes 'plot' ('plot' can be a synonym of 'secret plan').
'i' put inside 'plot' is 'PILOT'.
'for' is the link.
(Other definitions for pilot that I've seen before include "Trial broadcast" , "Navigate" , "Person in charge of aircraft" , "may be up in the clouds" , "Name of Mr Rochester's dog" .)