Fruit that's sour – in Oslo, especially (4)
I believe the answer is:
sloe
'fruit' is the definition.
(I've seen this before)
'in oslo especially' is the wordplay.
'in' says the answer is hidden in the clue.
'SLOE' is hidden in the letters of 'oslo especially'.
'that's sour' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for sloe that I've seen before include "Fruit often used for gin" , "Sour blue-black fruit" , "Black fruit" , "Sour blackthorn fruit" , "Fruit of the blackthorn does't sound fast" .)