Dressed like a chap with a cool head (4)
I believe the answer is:
clad
'dressed' is the definition.
(synonyms)
'a chap with a cool head' is the wordplay.
'a chap' becomes 'lad' ('lad' can be a synonym of 'chap').
'with' says to put letters next to each other.
'head' says to take the initial letters.
The initial letter of 'cool' is 'c'.
'lad' put after 'c' is 'CLAD'.
'like' is the link.
(Other definitions for clad that I've seen before include "Provided with covering" , "Clothed, covered" , "Cover with insulation" , "Decked" , "Dressed" .)