Barbarian companion's irrational feeling (5)
I believe the answer is:
hunch
'irrational feeling' is the definition.
(hunch can mean a feeling or suspicion)
'barbarian companion's' is the wordplay.
'barbarian' becomes 'hun' (as in Atilla the Hun).
'companion' becomes 'ch' (Companion of Honour).
'hun'+'ch'='HUNCH'
(Other definitions for hunch that I've seen before include "Idea based on intuition" , "Intuition, suspicion" , "Bend over" , "Guess -- sit with bad posture" , "Lump - intuitive feeling" .)