Vague idea held by Hamish, unchallenged (5)
I believe the answer is:
hunch
'vague idea' is the definition.
'held by hamish unchallenged' is the wordplay.
'held by' indicates the answer is hidden within the clue.
'HUNCH' can be found hidden inside 'hamish unchallenged'.
(Other definitions for hunch that I've seen before include "Guess -- sit with bad posture" , "Lump - intuitive feeling" , "Bend over" , "Intuitive guess" , "Intuition, suspicion" .)