Arrive right within brook (5)
I believe the answer is:
river
'brook' is the definition.
(thesaurus)
'arrive right within' is the wordplay.
'within' says the answer is hidden in the clue.
'RIVER' is hidden in the letters of 'arrive right'.
(Other definitions for river that I've seen before include "Slow-cooked meat dish" , "It flows" , "Eg, the Rhine" , "Dee, Exe or Wye, say" , "Natural flow of water" .)