Mad idiot picked up a jerk (7)
I believe the answer is:
lunatic
'mad' is the definition.
(I've seen this before)
'idiot picked up a jerk' is the wordplay.
'idiot' becomes 'lu' (I can't explain this - if you can you should give a lot more credence to this answer).
'picked up' says the letters should be written in reverse.
'a' becomes 'an'.
'jerk' becomes 'tic' (jerking movement by the body).
'an' written backwards gives 'na'.
'lu'+'na'+'tic'='LUNATIC'
(Other definitions for lunatic that I've seen before include "crazed?" , "Deranged" , "Imbecile" , "Nutcase" , "nut" .)