Second time in bank, behind closed doors (8)
I believe the answer is:
secretly
'behind closed doors' is the definition.
(thesaurus)
'second time in bank' is the wordplay.
'second' becomes 'sec' ('sec' can be a synonym of 'second').
'time' becomes 't'.
'in' is an insertion indicator.
'bank' becomes 'rely' (I've seen this before).
't' put within 'rely' is 'retly'.
'sec'+'retly'='SECRETLY'
(Other definitions for secretly that I've seen before include "privately" , "Undercover" , "under cover" , "Not openly" , "On the quiet" .)