Bird so hard to draw in river (6)
I believe the answer is:
thrush
'bird' is the definition.
(thrush is a kind of bird)
'so hard to draw in river' is the wordplay.
'so' becomes 'thus' (synonyms).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'to draw in' is an insertion indicator.
'river' becomes 'r' (abbreviation for river).
'thus'+'h'='thush'
'thush' placed around 'r' is 'THRUSH'.
(Other definitions for thrush that I've seen before include "Songbird - fungal infection" , "Mouth fungus or songbird" , "Brownish songbirds" , "Throstle" , "Singing bird or mouth-troat infection" .)