With a cool head, the young fellow dressed (4)
I believe the answer is:
clad
'dressed' is the definition.
('clothe' can be a synonym of 'dress')
'with a cool head the young fellow' is the wordplay.
'with' is a charade indicator (letters next to each other).
'head' indicates taking the first letters.
'the young fellow' becomes 'lad' (I've seen this before).
The first letter of 'cool' is 'c'.
'c' next to 'lad' is 'CLAD'.
(Other definitions for clad that I've seen before include "Provided with covering" , "Decked" , "Wearing" , "Clothed, covered" , "Attired" .)