Again accepts an island as high ground (4)
I believe the answer is:
rise
'high ground' is the definition.
(I've seen this in another clue)
'again accepts an island' is the wordplay.
'again' becomes 're' (prefix meaning 'again').
'accepts' means one lot of letters goes inside another.
'an island' becomes 'is' (abbreviation).
're' placed around 'is' is 'RISE'.
'as' acts as a link.
(Other definitions for rise that I've seen before include "Pay increase" , "Growth" , "Hill" , "Ascent" , "Go up" .)