Murphy drops old rubbish in river (6)
I believe the answer is:
potato
'murphy' is the definition.
(synonyms)
'drops old rubbish in river' is the wordplay.
'drops' says to take the initial letters (I've seen 'drop of' mean this).
'rubbish' becomes 'tat' (low-quality items or material).
'in' means one lot of letters goes inside another.
'river' becomes 'Po' (Italian river).
The initial letter of 'old' is 'o'.
'o'+'tat'='otat'
'otat' placed into 'po' is 'POTATO'.
(Other definitions for potato that I've seen before include "Plant tuber eaten as a vegetable" , "Tuber, popular in Ireland" , "Common vegetable" , "Starchy tuber" , "The vegetable is too apt, oddly" .)