Box with a cool head, at speed (5)
I believe the answer is:
crate
'box' is the definition.
(I've seen this before)
'a cool head at speed' is the wordplay.
'head' indicates taking the first letters.
'at' says to put letters next to each other.
'speed' becomes 'rate' (rate is a kind of speed**).
The initial letter of 'cool' is 'c'.
'c'+'rate'='CRATE'
'with' acts as a link.
(Other definitions for crate that I've seen before include "Dilapidated vehicle" , "Box for shipping" , "Trace to a large container" , "heap" , "Old car" .)