Tube train not popular with revolutionary American (7)
I believe the answer is:
trachea
'tube' is the definition.
(tube in the throat)
'train not popular with revolutionary american' is the wordplay.
'not' indicates named letters should be taken away.
'popular' becomes 'in' ('in' can mean trendy).
'with' means one lot of letters go next to another.
'revolutionary' becomes 'che' (Che Guevara).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'train' with 'in' removed is 'tra'.
'tra'+'che'+'a'='TRACHEA'
(Other definitions for trachea that I've seen before include "The formal windpipe" , "Tube carrying air to the lungs" , "membranous tube" , "Tube taking inhaled air from the larynx" , "get air supply through this" .)