Keen on crossing river -- it's a short passage (5)
I believe the answer is:
intro
'passage' is the definition.
(intro is a kind of passage)
'keen on crossing river it's a short' is the wordplay.
'keen on' becomes 'into' (I've seen this before).
'crossing' is an insertion indicator.
'it's a short' indicates taking the first letters.
The first letter of 'river' is 'r'.
'into' going around 'r' is 'INTRO'.
(Other definitions for intro that I've seen before include "Preamble" , "First bars of a musical composition" , "Early music piece" , "Lead-in" , "Preliminary section (of music) (colloq.)" .)