Left a nut, stupidly, in charge of a nut! (7)
I believe the answer is:
lunatic
'a nut' is the definition.
(I've seen this before)
'left a nut stupidly in charge' is the wordplay.
'left' becomes 'l' (common abbreviation**).
'stupidly' is an anagram indicator.
'in charge' becomes 'ic'.
'a'+'nut'='anut'
'anut' anagrammed gives 'unat'.
'l'+'unat'+'ic'='LUNATIC'
'of' is the link.
(Other definitions for lunatic that I've seen before include "Deranged" , "Foolish person" , "Type of fringe" , "Harebrained" , "Bats" .)