Finished float losing hydrogen (4)
I believe the answer is:
over
'finished' is the definition.
(similar in meaning)
'float losing hydrogen' is the wordplay.
'float' becomes 'hover' (I've seen this in another clue).
'losing' suggests deleting specific letters.
'hydrogen' becomes 'H' (chemical symbol).
'hover' with 'h' taken away is 'OVER'.
(Other definitions for over that I've seen before include "Having gone < to America" , "settled" , "See 23" , "Finished, done" , "In the sky?" .)