Confused MP, so ill, left shoe (8)
I believe the answer is:
plimsoll
'shoe' is the definition.
(plimsoll is a kind of shoe)
'confused mp so ill left' is the wordplay.
'confused' is an anagram indicator.
'left' becomes 'l' (common abbreviation).
'mp'+'so'+'ill'='mpsoill'
'mpsoill' is an anagram of 'plimsol'.
'plimsol'+'l'='PLIMSOLL'
(Other definitions for plimsoll that I've seen before include "Sports shoe; a line" , "This line on ship's side measures degree of loading" , "The ........ line is a loading mark on ship''s side" , "Pump social reformer" , "Light gym shoe and waterline" .)