Insubstantial and small, containing nitrogen (5)
I believe the answer is:
tinny
'insubstantial' is the definition.
(I've seen this before)
'small containing nitrogen' is the wordplay.
'small' becomes 'tiny' (similar in meaning).
'containing' means one lot of letters goes inside another.
'nitrogen' becomes 'N' (N is the chemical symbol for nitrogen).
'tiny' going around 'n' is 'TINNY'.
'and' acts as a link.
(Other definitions for tinny that I've seen before include "With thin metallic sound" , "Lacking resonance" , "Metallic-sounding" , "Sounding thin and metallic" , "Made of poor metal" .)