Strange ooze absorbing nitrogen gas (5)
I believe the answer is:
ozone
'gas' is the definition.
(I've seen this before)
'strange ooze absorbing nitrogen' is the wordplay.
'strange' indicates anagramming the letters.
'absorbing' is an insertion indicator.
'nitrogen' becomes 'N' (N is the chemical symbol for nitrogen).
'ooze' with letters rearranged gives 'ozoe'.
'ozoe' placed around 'n' is 'OZONE'.
(Other definitions for ozone that I've seen before include "Allotrope of oxygen" , "A gas; threatened layer in air" , "Gas; bracing sea air" , "Form of oxygen in earth's upper atmosphere - there's a hole in it" , "Less common form of oxygen" .)