Kelvin casual, playing horn (6)
I believe the answer is:
klaxon
'horn' is the definition.
(klaxon is a kind of horn)
'kelvin casual playing' is the wordplay.
'kelvin' becomes 'K' (abbreviation for the unit of temperature).
'casual' becomes 'lax' (I've seen this before).
'playing' becomes 'on'.
'k'+'lax'+'on'='KLAXON'
(Other definitions for klaxon that I've seen before include "Blaring horn" , "warning device" , "A kind of loud horn" , "Warning horn" , "Old car horn" .)