Is penny invested in stupid deal (8)
I believe the answer is:
dispense
'deal' is the definition.
('dispense' can be a synonym of 'deal')
'is penny invested in stupid' is the wordplay.
'penny' becomes 'p'.
'invested in' means one lot of letters goes inside another.
'stupid' becomes 'dense' ('dense' can be similar in meaning to 'stupid').
'is'+'p'='isp'
'isp' inserted into 'dense' is 'DISPENSE'.
(Other definitions for dispense that I've seen before include "Deal out or distribute" , "Enforce (justice)" , "Prepare and give out, as chemist does medicines" , "Hand out (medicine)" , "Supply (as chemist)" .)