Still a challenge to cross river (2,4)
I believe the answer is:
at rest
'still' is the definition.
(synonyms)
'a challenge to cross river' is the wordplay.
'challenge' becomes 'test' (I've seen this in another clue).
'to cross' means one lot of letters goes inside another (some letters cross or go round others).
'river' becomes 'r' (abbreviation).
'test' placed around 'r' is 'trest'.
'a'+'trest'='AT REST'
(Other definitions for at rest that I've seen before include "calm" , "late" , "In repose" , "Not moving, free from anxiety" , "Stopped" .)