Stuff beginning to rot in river at Cambridge (4)
I believe the answer is:
cram
'stuff' is the definition.
(synonyms)
'beginning to rot in river at cambridge' is the wordplay.
'beginning to' says to take the initial letters.
'in' is an insertion indicator.
'river at cambridge' becomes 'cam' (River Cam).
The initial letter of 'rot' is 'r'.
'r' inserted inside 'cam' is 'CRAM'.
(Other definitions for cram that I've seen before include "Stuff full" , "Pack in or study hard" , "prepare for exam" , "Press together" , "Study intensively" .)