Fine upset corporation importing chlorine (5)
I believe the answer is:
mulct
'fine' is the definition.
(synonyms)
'upset corporation importing chlorine' is the wordplay.
'upset' shows that the letters should be reversed in order.
'corporation' becomes 'tum' ('corporation' is a humorous term for stomach fat).
'importing' is an insertion indicator.
'chlorine' becomes 'Cl' (Cl is the chemical symbol for chlorine).
'tum' placed around 'cl' is 'tclum'.
'tclum' in reverse letter order is 'MULCT'.
(Other definitions for mulct that I've seen before include "Extract money from (someone) by fine or taxation" , "Fleece" , "Diddle" , "Cheat, defraud" , "Extract money (from someone) by deceit" .)