Day you will invest a thousand in bank (4)
I believe the answer is:
dyke
'bank' is the definition.
'day you will invest a thousand in' is the wordplay.
'day' becomes 'd' (abbreviation for day).
'you' becomes 'ye' (historical version of 'you').
'will' means one lot of letters go next to another (I've seen this in other clues).
'invest a thousand' becomes 'k' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'd'+'ye'='dye'
'dye' going around 'k' is 'DYKE'.
Can you help me to learn more?
(Other definitions for dyke that I've seen before include "Anti-flood ditch/wall" , "Earthwork; watercourse" , "Barrier to prevent flooding" , "Barrier against water" , "Flood defence" .)