Star lives up tiny narrow passage (5)
I believe the answer is:
sinus
'tiny narrow passage' is the definition.
'sinus' can be an answer for 'passage' (sinus is a kind of passage). I'm not certain of the 'tiny narrow' bit.
'star lives up' is the wordplay.
'star' becomes 'sun' (the Sun is a star).
'lives' becomes 'is' ('be' can be a synonym of 'live').
'up' says the letters should be written backwards (in down clue: letters go upwards).
'sun'+'is'='sunis'
'sunis' backwards is 'SINUS'.
Can you help me to learn more?
(Other definitions for sinus that I've seen before include "Channel between brain and skull" , "Air cavity in the cranial bones" , "Cavity within a bone" , "Cavity in the facial bones" , "Space in temple" .)