Pure river spirit found in small container (8)
I believe the answer is:
virginal
'pure' is the definition.
(synonyms)
'river spirit found in small container' is the wordplay.
'river' becomes 'r'.
'spirit' becomes 'gin' (I've seen this before).
'found in' indicates putting letters inside.
'small container' becomes 'vial'.
'r'+'gin'='rgin'
'rgin' inserted within 'vial' is 'VIRGINAL'.
(Other definitions for virginal that I've seen before include "Untouched - spinet" , "Pure - very early keyboard instrument" , "Rectangular harpsichord" , "Antique keyboard" , "in immaculate condition" .)