Thinking about one section treating ill (8)
I believe the answer is:
misusing
'treating ill' is the definition.
'thinking about one section' is the wordplay.
'thinking' becomes 'musing' (musing is a kind of thinking**).
'about' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'section' becomes 's' (legal abbreviation).
'i'+'s'='is'
'musing' going around 'is' is 'MISUSING'.
(Other definitions for misusing that I've seen before include "Maltreating" , "Perverting, misapplying" .)