Set aside search in spot by river (9)
I believe the answer is:
sequester
'set aside' is the definition.
(to sequester oneself is to hide oneself away)
'search in spot by river' is the wordplay.
'search' becomes 'quest' (quest is a kind of search).
'in' means one lot of letters goes inside another.
'spot by' becomes 'see' (I've seen this in another clue. I am not sure about the 'by' bit.).
'river' becomes 'r' (abbreviation for river).
'quest' put inside 'see' is 'sequeste'.
'sequeste'+'r'='SEQUESTER'
(Other definitions for sequester that I've seen before include "Set aside" , "Seize" , "Remove; confiscate (property)" , "Isolate or hide away (something or someone)" , "Separate" .)