River also rising, trapping small perch (5)
I believe the answer is:
roost
'perch' is the definition.
(I know that perch can be written as roost)
'river also rising trapping small' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'also' becomes 'too' (synonyms).
'rising' is a reversal indicator (going up in a down clue).
'trapping' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'too' in reverse letter order is 'oot'.
'oot' going around 's' is 'oost'.
'r'+'oost'='ROOST'
(Other definitions for roost that I've seen before include "(Of birds) settle for rest" , "Fowl's perch" , "Settle down (on a branch?)" , "Place for sleeping birds" , "Perch on a branch" .)