Bird heard when crossing river (4)
I believe the answer is:
wren
'bird' is the definition.
(wren is a kind of bird)
'heard when crossing river' is the wordplay.
'heard' becomes 'wen' (I can't justify this - if you can you should believe this answer much more).
'when crossing' indicates putting letters inside.
'river' becomes 'r' (abbreviation for river).
'wen' enclosing 'r' is 'WREN'.
(Other definitions for wren that I've seen before include "A small garden bird" , "18 [SWAN] 's little cousin" , "Small singer" , "Troglodytes troglodytes" , "Architect of St Paul's Cathedral" .)