Tube train less popular? Long to board it (7)
I believe the answer is:
trachea
'tube' is the definition.
(tube in the throat)
'long to board it' is the wordplay.
'long' becomes 'ache' (to long for something is to ache for it).
'to board' is an insertion indicator (inserted letters come on board).
'it' becomes 'tra' (I am not sure about this - if you are sure you should believe this answer much more).
'ache' put into 'tra' is 'TRACHEA'.
'train less popular?' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for trachea that I've seen before include "body part" , "get air supply through this" , "Breathing tube" , "The formal windpipe" , "air passage" .)