River Ouse's outsides removed by thinker (5)
I believe the answer is:
camus
'thinker' is the definition.
(philosopher and author Albert Camus)
'river ouse's outsides removed by' is the wordplay.
'river' becomes 'cam' (river in Cambridge, England).
'outsides removed' says to take the centre (the outside letters are removed).
'by' means one lot of letters go next to another.
The central letters of 'ouse' are 'us'.
'cam'+'us'='CAMUS'
(Other definitions for camus that I've seen before include "Albert -, French writer" , "20th century French philosopher and novelist" , "He wrote" , "Albert -, Fr. writer" , "he pondered the human condition" .)