So immersed in river yesterday (4)
I believe the answer is:
very
'so' is the definition.
(I've seen this in another clue)
'immersed in river yesterday' is the wordplay.
'immersed in' indicates the answer is hidden within the clue.
'VERY' can be found hidden inside 'river yesterday'.
(Other definitions for very that I've seen before include "Extremely, in a high degree" , "Real" , "jolly British!" , "Exactly, really" , "Sort of light" .)