Lark rising in the afternoon over river (4)
I believe the answer is:
romp
'lark' is the definition.
('romp' can be a synonym of 'lark')
'rising in the afternoon over river' is the wordplay.
'rising' shows that the letters should be reversed in order (going up in a down clue).
'in the afternoon' becomes 'pm'.
'over' becomes 'o' (cricket abbreviation**).
'river' becomes 'r'.
'pm'+'o'+'r'='pmor'
'pmor' back-to-front is 'ROMP'.
(Other definitions for romp that I've seen before include "Boisterous prank" , "Frisk - gambol" , "Lively play during prom" , "Play in a lively way, like children" , "Frolic" .)