Heading for Camden, tear off to catch the Tube (8)
I believe the answer is:
catheter
'tube' is the definition.
(catheter is a kind of tube)
'heading for camden tear off to catch the' is the wordplay.
'heading for' indicates taking the first letters.
'off' indicates an anagram.
'to catch' means one lot of letters goes inside another.
The initial letter of 'camden' is 'c'.
'tear' is an anagram of 'ater'.
'ater' going around 'the' is 'atheter'.
'c'+'atheter'='CATHETER'
(Other definitions for catheter that I've seen before include "Long, thin flexible tube for medical use" , "Trace the strange tube used medically" , "Medical tube to remove waste from body - heart etc (anag)" , "Thin flexible tube to withdraw fluids from the body" .)