Some thinner vessel, bottle (5)
I believe the answer is:
nerve
'bottle' is the definition.
(both can mean courage)
'some thinner vessel' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'NERVE' can be found hidden inside 'thinner vessel'.
(Other definitions for nerve that I've seen before include "displaying self-possession" , "Brass neck" , "Courage, like bodily fibre" , "Sensory organ; bottle" , "Coolness under fire" .)