Stuff credit a source of misery! (4)
I believe the answer is:
cram
'stuff' is the definition.
(cramming is a kind of stuffing)
'credit a source of misery' is the wordplay.
'credit' becomes 'cr'.
'source of' suggests taking the first letters.
The first letter of 'misery' is 'm'.
'cr'+'a'+'m'='CRAM'
(Other definitions for cram that I've seen before include "Swot (for exam)" , "Revise hard" , "prepare hastily for the paper" , "Press together" , "Stuff (facts for exam)" .)