Something fishy a politician kept in shed (7)
I believe the answer is:
halibut
'something fishy' is the definition.
(halibut is a kind of fish)
'a politician kept in shed' is the wordplay.
'politician' becomes 'lib'.
'kept in' means one lot of letters goes inside another.
'shed' becomes 'hut' (I've seen this in another clue).
'a'+'lib'='alib'
'alib' inserted inside 'hut' is 'HALIBUT'.
(Other definitions for halibut that I've seen before include "Large edible flatfish" , "Fishy sort" , "Large flatfish used as food" , "Marine food fish" , "Utah lib (anag)" .)