Look firm - but is mad! (4)
I believe the answer is:
loco
'mad' is the definition.
(thesaurus)
'look firm' is the wordplay.
'look' becomes 'lo'.
'firm' becomes 'co' (a firm is a company).
'lo'+'co'='LOCO'
'but is' is the link.
(Other definitions for loco that I've seen before include "Slangily crazy" , "In ... parentis, in place of parents" , "Potty" , "Engine (abbrev.)" , "with a screw loose?" .)