Quiet before long envelops northern river (7)
I believe the answer is:
shannon
'river' is the definition.
(I've seen this before)
'quiet before long envelops northern' is the wordplay.
'quiet' becomes 'sh' ('sh' is used to tell someone to be quiet).
'before long' becomes 'anon' (anon can mean soon).
'envelops' means one lot of letters goes inside another (I've seen this in other clues).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'anon' going around 'n' is 'annon'.
'sh'+'annon'='SHANNON'
(Other definitions for shannon that I've seen before include "River in Irish Republic" , "Have some nosh, Nan, down by the river" , "Major Irish river" , "One who rendered runaway" , "Irish banker" .)