Evil bank acquires church, honestly! (9)
I believe the answer is:
sincerely
'honestly' is the definition.
('sincerely' can be a synonym of 'honestly')
'evil bank acquires church' is the wordplay.
'evil' becomes 'sin' (I've seen this before).
'bank' becomes 'rely' (I've seen this before).
'acquires' indicates putting letters inside.
'church' becomes 'ce' (abbreviation for Church of England).
'sin'+'rely'='sinrely'
'sinrely' enclosing 'ce' is 'SINCERELY'.
(Other definitions for sincerely that I've seen before include "Last word in letter" , "Genuinely, honestly" , "Really" , "With genuine feelings" , "the last word before signing" .)