I cry a river, which indicates constant pressure (6)
I believe the answer is:
isobar
'i cry a river which indicates constant pressure' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'i cry a river' is the wordplay.
'cry' becomes 'sob' (sobbing is a kind of crying).
'river' becomes 'r' (abbreviation for river).
'i'+'sob'+'a'+'r'='ISOBAR'
Can you help me to learn more?
(Other definitions for isobar that I've seen before include "Map line showing atmosphere pressure" , "It connects points of equal barometric pressure" , "Line on map indicating barometric pressure" , "Line joining places of equal barometric pressure" , "A Boris (anag.)" .)