Spots expert inhaling nitrogen (4)
I believe the answer is:
acne
'spots' is the definition.
(skin condition causing spots)
'expert inhaling nitrogen' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'inhaling' indicates putting letters inside.
'nitrogen' becomes 'N' (N is the chemical symbol for nitrogen).
'ace' placed around 'n' is 'ACNE'.
(Other definitions for acne that I've seen before include "Cane for the teenager?" , "Skin affliction" , "Skin complaint" , "Inflammation of glands in skin" , "Spotty skin condition" .)