A short time in river Test (7)
I believe the answer is:
examine
'test' is the definition.
(synonyms)
'a short time in river' is the wordplay.
'short time' becomes 'min' (abbreviation for minute).
'in' indicates putting letters inside.
'river' becomes 'exe' (River Exe).
'a'+'min'='amin'
'amin' inserted into 'exe' is 'EXAMINE'.
(Other definitions for examine that I've seen before include "Inspect; question in court" , "Formally question (law)" , "Inspect, study" , "Investigate" , "Inspect, peruse closely" .)