Naive exploit crossing river ending in tragedy (4-4)
I believe the answer is:
dewy-eyed
'naive' is the definition.
(I've seen this before)
'exploit crossing river ending in tragedy' is the wordplay.
'exploit' becomes 'deed' (both can mean a feat or escapade).
'crossing' is an insertion indicator (to cross can mean to go over or around).
'river' becomes 'wye' (I've seen this in other clues).
'ending in' indicates one should take the final letters.
The last letter of 'tragedy' is 'y'.
'wye'+'y'='wyey'
'deed' enclosing 'wyey' is 'DEWY-EYED'.
(Other definitions for dewy-eyed that I've seen before include "Innocent" , "Exhibiting childlike credulity" , "Affected by nostalgic or sentimental tears?" , "Sentimental with moist optics" , "Sentimental or innocently trusting, being ocularly moist?" .)