Nasty trick, inhaling hydrogen at the same time (9)
I believe the answer is:
meanwhile
'at the same time' is the definition.
('meanwhile' can be a synonym of 'at the same time')
'nasty trick inhaling hydrogen' is the wordplay.
'nasty' becomes 'mean' ('mean' can be similar in meaning to 'nasty').
'trick' becomes 'wile' (I've seen this before**).
'inhaling' means one lot of letters goes inside another.
'hydrogen' becomes 'h' (H is the chemical symbol for hydrogen).
'wile' enclosing 'h' is 'while'.
'mean'+'while'='MEANWHILE'
(Other definitions for meanwhile that I've seen before include "On the other hand" , "simultaneously" , "In the intervening time" , "in intervening period" , "In the interim" .)