Second time in bank, behind closed doors (8)
I believe the answer is:
Here is my best explanation:
'behind closed doors' is the definition.
'second time in bank' is the subsidiary indication.
'second' becomes 'sec' ('sec' can be a synonym of 'second').
'time' becomes 't'.
'in' indicates putting letters inside.
'bank' becomes 'rely' (I've seen this before**).
't' put within 'rely' is 'retly'.
(Other definitions for secretly that I've seen before include "privately", "Undercover", "under cover", "On the quiet", "Covertly", "behind closed doors", "in disguise?", "without telling anyone", "in camera" and "On the QT".)