Influx of hydrogen after one runs out (6)
I believe the answer is:
inrush
'influx' is the definition.
(inrush is a kind of influx)
'hydrogen after one runs out' is the wordplay.
'hydrogen' becomes 'H' (chemical symbol).
'after' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'runs' anagrammed gives 'nrus'.
'i'+'nrus'='inrus'
'h' after 'inrus' is 'INRUSH'.
'of' is the link.
(Other definitions for inrush that I've seen before include "Overwhelming flow" , "Sudden move inwards" , "Rapid flow from outside" , "Internal thrust" , "gaining sudden entry" .)