Tube of paintings knocked over by red American (7)
I believe the answer is:
trachea
'tube' is the definition.
(trachea is a kind of tube)
'paintings knocked over by red american' is the wordplay.
'paintings' becomes 'art' (I've seen this before).
'knocked over' says the letters should be written backwards.
'by' means one lot of letters go next to another.
'red' becomes 'che' (Che Guevara the Marxist revolutionary).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'art' back-to-front is 'tra'.
'tra'+'che'+'a'='TRACHEA'
'of' is the link.
(Other definitions for trachea that I've seen before include "membranous tube" , "Tube taking inhaled air from the larynx" , "Breathing tube" , "airy passage" , "get air supply through this" .)