Ran into man inhaling oxygen very fast (8)
I believe the answer is:
meteoric
'very fast' is the definition.
(I've seen this before)
'ran into man inhaling oxygen' is the wordplay.
'ran into' becomes 'met' (I've seen this in another clue).
'man' becomes 'eric' (I've seen this before).
'inhaling' is an insertion indicator.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'eric' enclosing 'o' is 'eoric'.
'met'+'eoric'='METEORIC'
(Other definitions for meteoric that I've seen before include "Spectacular sort of rise" , "Dazzling and rapid, rise say" , "with brilliant speed" , "Remarkably quick" , "Of a shooting star; remarkably fast" .)