Invest in a unit of the open market (7)
I believe the answer is:
install
'invest' is the definition.
(I've seen this before)
'in a unit of the open market' is the wordplay.
'a unit of the open market' becomes 'stall' (I can't explain this - if you can you should give a lot more credence to this answer).
'in'+'stall'='INSTALL'
Can you help me to learn more?
(Other definitions for install that I've seen before include "Place (something) in its working position" , "Place in position or rank" , "Place in position, establish" , "Put into position, ready for use" , "Fix (equipment) in position" .)