Constant smoker worried about inhaling nitrogen (4)
I believe the answer is:
etna
'constant smoker' is the definition.
(I've seen this before)
'worried about inhaling nitrogen' is the wordplay.
'worried' becomes 'ate' (something worrying may eat someone up).
'about' says the letters should be written in reverse.
'inhaling' is an insertion indicator.
'nitrogen' becomes 'N' (N is the chemical symbol for nitrogen).
'ate' back-to-front is 'eta'.
'eta' going around 'n' is 'ETNA'.
(Other definitions for etna that I've seen before include "Impressive mount" , "Sicilian volcano" , "Dangerous peak" , "From a dangerous height" , "Threatening mountain" .)