Semi-aquatic mammal picked up north of Washington capital (6)
I believe the answer is:
ottawa
'capital' is the definition.
(Ottawa is an example)
'semi-aquatic mammal picked up north of washington' is the wordplay.
I cannot quite see how this works, but
'of' could be 'o' and 'o' is present in the answer.
'washington' could be 'wa' (abbreviation for the US state) and 'wa' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for ottawa that I've seen before include "somewhere in Canada" , "Canadian capital" , "The capital of Canada (6)" , "capital city" , "Canadian city" .)