Cut oxygen, say, then hydrogen (4)
I believe the answer is:
gash
'cut' is the definition.
(I've seen this before)
'oxygen say then hydrogen' is the wordplay.
'oxygen say' becomes 'gas' (oxygen is a kind of gas).
'then' means one lot of letters go next to another.
'hydrogen' becomes 'h' (H is the chemical symbol for hydrogen).
'gas'+'h'='GASH'
(Other definitions for gash that I've seen before include "injury" , "Cut; waste (colloq.)" , "extra" , "score heavily" , "Long deep slash" .)