Stew at initially onerous and difficult problem (3,6)
I believe the answer is:
hot potato
'difficult problem' is the definition.
(a hot potato is a difficult or unwanted problem)
'stew at initially onerous' is the wordplay.
'stew' becomes 'hotpot' (I've seen this before).
'initially' suggests taking the first letters.
The initial letter of 'onerous' is 'o'.
'hotpot'+'at'+'o'='HOT POTATO'
'and' is the link.
(Other definitions for hot potato that I've seen before include "Problem that's painful to grasp" , "contentious question" , "tricky issue?" , "controversial political issue?" , "taken with pinch of salt?" .)