About to capture hydrogen in river (5)
I believe the answer is:
rhine
'river' is the definition.
(I know that Rhine is an river)
'about to capture hydrogen in' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'to capture' indicates putting letters inside.
'hydrogen' becomes 'H' (H is the chemical symbol for hydrogen).
'h'+'in'='hin'
're' going around 'hin' is 'RHINE'.
(Other definitions for rhine that I've seen before include "Ring river (Wagner)" , "Great European river with Lorelei Rock" , "River at Bonn, Koln" , "Great German river with Lorelei Rock" , "River flowing from Switzerland, through Germany, to the North Sea" .)