Nocturnal primate girl observed crossing river (5)
I believe the answer is:
loris
'nocturnal primate' is the definition.
(I've seen this before)
'girl observed crossing river' is the wordplay.
I cannot really understand how this works, but
'river' could be 'r' (abbreviation for river) and 'r' is located in the answer.
'girl' could be 'lois' and 'lois' is located in the remaining letters.
No letters remain.
This explanation may well be incorrect...
(Other definitions for loris that I've seen before include "Slow-moving nocturnal primate" , "lemur" , "Short-tailed mammal" , "Nocturnal primate of Southeast Asia (usually slender or slow)" , "Small nocturnal primate" .)