Save when banks leave fund a smaller amount (6)
I believe the answer is:
unless
'save' is the definition.
(thesaurus)
'banks leave fund a smaller amount' is the wordplay.
'banks leave' indicates the central letters (the banks are the letters on either side).
'a smaller amount' becomes 'less' (I've seen this before).
The central letters of 'fund' are 'un'.
'un'+'less'='UNLESS'
'when' is the link.
(Other definitions for unless that I've seen before include "Except, save" , "Except on the condition that" , "Conditional on" , "Though" , "bar" .)