Low point when free article is sent back (5)
I believe the answer is:
nadir
'low point' is the definition.
(I've seen this before)
'free article is sent back' is the wordplay.
'free' becomes 'rid' (to be free of something is to be rid of it).
'article' becomes 'an' (English indefinite article).
'is sent back' says the letters should be written backwards.
'rid'+'an'='ridan'
'ridan' back-to-front is 'NADIR'.
'when' acts as a link.
(Other definitions for nadir that I've seen before include "The lowest point of anything" , "Rock bottom" , "pits" , "deep despair" , "You can't sink lower!" .)