Reliant upon changing for convenience (7)
I believe the answer is:
latrine
'convenience' is the definition.
(latrine is a kind of convenience)
'reliant upon changing' is the wordplay.
'upon changing' is an anagram indicator.
'reliant' is an anagram of 'LATRINE'.
'for' acts as a link.
(Other definitions for latrine that I've seen before include "Gentlemen's" , "public toilet" , "office" , "Army lavatory" , "Camp toilet" .)