Developed into flying insect around river (6)
'developed into' is the definition.
(I've seen this before)
'flying insect around river' is the wordplay.
'flying insect' becomes 'bee' (I've seen this before).
'around' indicates putting letters inside.
'river' becomes 'cam' (I've seen this before).
'bee' going around 'cam' is 'BECAME'.
(Other definitions for became that I've seen before include "Began to be", "Grew", "Underwent a change", "Turned into", "Suited".)