Does nothing but concoct lies about the head of department (5)
I believe the answer is:
idles
'does nothing' is the definition.
(I've seen this before)
'concoct lies about the head of department' is the wordplay.
'concoct' indicates anagramming the letters (I've seen 'concocted' mean this).
'about' is an insertion indicator.
'the head of' indicates taking the first letters.
The initial letter of 'department' is 'd'.
'lies' is an anagram of 'iles'.
'iles' placed around 'd' is 'IDLES'.
'but' acts as a link.
(Other definitions for idles that I've seen before include "Lazes about, just ticks over" , "Avoids working" , "Lazes around" , "Of engine, just ticks over in neutral" , "Does little, just ticks over" .)