Frogman comes across ship with stern centrally split (8)
I believe the answer is:
dissever
'split' is the definition.
('dissever' can be a synonym of 'split')
'frogman comes across ship with stern centrally' is the wordplay.
'frogman' becomes 'diver' ('diver' can be a synonym of 'frogman').
'comes across' is an insertion indicator.
'ship' becomes 'ss'.
'with' is a charade indicator (letters next to each other).
'centrally' says to take the centre.
The middle letter of 'stern' is 'e'.
'ss'+'e'='sse'
'diver' going around 'sse' is 'DISSEVER'.
(Other definitions for dissever that I've seen before include "split apart" , "Cut apart" , "divide into parts" .)