Liquid upset over iron tool (5)
I believe the answer is:
knife
'tool' is the definition.
(knife is a kind of tool)
'liquid upset over iron' is the wordplay.
'liquid' becomes 'ink' (ink is a kind of liquid**).
'upset' says the letters should be written backwards.
'over' says to put letters next to each other.
'iron' becomes 'fe' (Fe is the chemical symbol for iron).
'ink' in reverse letter order is 'kni'.
'kni'+'fe'='KNIFE'
(Other definitions for knife that I've seen before include "Use this to carve" , "Table implement" , "Edge tool" , "Lethal weapon" , "Cut" .)