In truth, bank will be occupied by a student (6)
I believe the answer is:
really
'in truth' is the definition.
('really' can be a synonym of 'in truth')
'bank will be occupied by a student' is the wordplay.
'bank' becomes 'rely' (I've seen this before).
'will be occupied by' means one lot of letters goes inside another.
'student' becomes 'l' (as in L-plates for learner drivers).
'a'+'l'='al'
'rely' placed around 'al' is 'REALLY'.
(Other definitions for really that I've seen before include "without a doubt" , "surprised expression" , "as a matter of fact" , "In truth - join forces again" , "Actually, in fact" .)