The companion is off colour - a cold (5)
I believe the answer is:
chill
'a cold' is the definition.
(chill can mean cold)
'the companion is off colour' is the wordplay.
'the companion' becomes 'ch' (Companion of Honour).
'is off colour' becomes 'ill' (off-colour can mean unwell).
'ch'+'ill'='CHILL'
(Other definitions for chill that I've seen before include "minor illness" , "This disheartens" , "Make cool" , "Slang injunction to relax - it's cold" , "Iciness" .)