Shorten it do, stupid! (8)
I believe the answer is:
condense
'shorten' is the definition.
(condensing is a kind of shortening)
'do stupid' is the wordplay.
'do' becomes 'con' (both can mean to defraud).
'stupid' becomes 'dense' (similar in meaning).
'con'+'dense'='CONDENSE'
'it' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for condense that I've seen before include "Change from vapour to liquid" , "Abbreviate" , "precis" , "contract?" , "Send once (anag.)" .)