Concerned about river (4)
I believe the answer is:
erne
'river' is the definition.
(I've seen this before)
'concerned about' is the wordplay.
'about' indicates a hidden word (the clue goes around or about the answer).
'ERNE' can be found hidden inside 'concerned'.
(Other definitions for erne that I've seen before include "Irish river - sea eagle" , "Island-studded lake in Fermanagh" , "bird of prey" , "River and lake of the North" , "European sea eagle" .)