Humour a companion - a charmer! (5)
I believe the answer is:
witch
'a charmer' is the definition.
(someone who charms)
'humour a companion' is the wordplay.
'humour' becomes 'wit' (synonyms).
'a companion' becomes 'ch' (Companion of Honour).
'wit'+'ch'='WITCH'
(Other definitions for witch that I've seen before include "charming female" , "Broomstick riderof children's tales" , "Hag, crone" , "Cauldron-stirring hag" , "Woman using magic" .)