River feature’s made-up detail I ignored (5)
I believe the answer is:
delta
'river feature's' is the definition.
(I've seen this in another clue)
'up detail i ignored' is the wordplay.
'up' indicates an anagram (letters get chucked up).
'ignored' suggests deleting specific letters.
'detail' with 'i' taken out is 'detal'.
'detal' is an anagram of 'DELTA'.
'made' is the link.
(Other definitions for delta that I've seen before include "Area of distributaries at a river mouth" , "Letter - where river divides" , "Fourth Greek letter? - area at river mouth" , "Fourth letter of the ancient Greek alphabet: river feature" , "Area of alluvial deposit" .)