Slow to contain river in loo (7)
I believe the answer is:
latrine
'loo' is the definition.
(I have seen 'Military loo' mean 'latrine' so perhaps 'loo' could also mean 'latrine')
'slow to contain river in' is the wordplay.
'slow' becomes 'late' (I've seen this in other clues).
'to contain' means one lot of letters goes inside another.
'river' becomes 'r' (abbreviation for river).
'r'+'in'='rin'
'late' enclosing 'rin' is 'LATRINE'.
(Other definitions for latrine that I've seen before include "Reliant on military loo" , "An army loo" , "office" , "Lavatory (in barracks)" , "Camp toilet" .)