Lied terribly to cover up extremely dim-witted swindle (6)
I believe the answer is:
diddle
'swindle' is the definition.
(synonyms)
'lied terribly to cover up extremely dim-witted' is the wordplay.
'terribly' is an anagram indicator.
'to cover up' is an insertion indicator.
'extremely dim-witted' becomes 'dd' (I can't explain this - if you can you should believe this answer much more).
'lied' is an anagram of 'dile'.
'dile' going around 'dd' is 'DIDDLE'.
Can you help me to learn more?
(Other definitions for diddle that I've seen before include "Slangily cheat or swindle" , "Slang for cheat or swindle - nearly fiddle" , "Con - do - lidded (anag)" , "Did, and led out to cheat" , "Fleece" .)