Poor MP, so ill, left with pump (8)
I believe the answer is:
plimsoll
'pump' is the definition.
(I've seen this before)
'poor mp so ill left' is the wordplay.
'poor' is an anagram indicator.
'left' becomes 'l' (common abbreviation).
'mp'+'so'+'ill'='mpsoill'
'mpsoill' with letters rearranged gives 'plimsol'.
'plimsol'+'l'='PLIMSOLL'
'with' acts as a link.
(Other definitions for plimsoll that I've seen before include "Mark on ship's side to indicate limit of loading" , "Pump social reformer" , "Marine line" , "footwear?" , "Shipping legislator" .)