The garment for many a company head (3)
I believe the answer is:
mac
'the garment' is the definition.
(mac is a kind of garment)
'many a company head' is the wordplay.
'many' becomes 'm' (Roman numeral for a thousand).
'head' says to take the initial letters.
The first letter of 'company' is 'c'.
'm'+'a'+'c'='MAC'
'for' acts as a link.
(Other definitions for mac that I've seen before include "It may help you keep dry" , "Raincoat (colloq.)" , "Waterproof raincoat, in short" , "Overcoat" , "one used in shower?" .)