Nitrogen in unopened pickle is concerning (2,2)
I believe the answer is:
in re
'is concerning' is the definition.
(short for 'in regards to' ie concerning a topic)
'nitrogen in unopened pickle' is the wordplay.
I cannot quite understand how this works, but
'nitrogen' could be 'n' (N is the chemical symbol for nitrogen) and 'n' is found within the answer.
The remaining letters 'ire' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for in re that I've seen before include "In the matter of (Lat.)" , "Concerning (L)" , "With regard to" .)