Force old banks in Berne to save 51 thousand pounds (6)
I believe the answer is:
oblige
'force' is the definition.
(I know that force is a kind of oblige)
'old banks in berne to save 51 thousand pounds' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'banks in' means to remove the middle letters (the edges or banks of the word).
'to save' is an insertion indicator.
'51' becomes 'LI' (Roman numerals).
'thousand pounds' becomes 'G' (G can informally mean a grand).
'berne' with its middle removed is 'be'.
'li'+'g'='lig'
'be' placed around 'lig' is 'blige'.
'o'+'blige'='OBLIGE'
(Other definitions for oblige that I've seen before include "serve" , "Compel legally, morally" , "Compel, force" , "press!" , "Compel (someone to do something)" .)