Neglect turning river into island (5)
I believe the answer is:
timor
'island' is the definition.
(I've seen this before)
'neglect turning river' is the wordplay.
'neglect' becomes 'omit' (synonyms).
'turning' is a reversal indicator.
'river' becomes 'r' (abbreviation for river).
'omit' back-to-front is 'timo'.
'timo'+'r'='TIMOR'
'into' is the link.
(Other definitions for timor that I've seen before include "SE Asian island" , "Indonesian island" , "Island of the Malay Archipelago" , "East Indies island NW from Darwin" , "Eastern island" .)