Influx of hydrogen after one runs out (6)
I believe the answer is:
inrush
'influx' is the definition.
(I know that inrush is a type of influx)
'hydrogen after one runs out' is the wordplay.
'hydrogen' becomes 'H' (H is the chemical symbol for hydrogen).
'after' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'out' indicates an anagram (out can mean wrong or inaccurate).
'runs' with letters rearranged gives 'nrus'.
'i'+'nrus'='inrus'
'h' after 'inrus' is 'INRUSH'.
'of' acts as a 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" .)