Rugby quartet included allowed stream (7)
I believe the answer is:
rivulet
'stream' is the definition.
(I know that rivulet is a type of stream)
'rugby quartet included allowed' is the wordplay.
'rugby' becomes 'RU'.
'quartet' becomes 'IV' (Roman numerals for four).
'included' is an insertion indicator.
'allowed' becomes 'let' (to let is to allow or permit).
'ru' placed around 'iv' is 'rivu'.
'rivu'+'let'='RIVULET'
(Other definitions for rivulet that I've seen before include "Vile rut for small stream" , "fast stream" , "Brook" , "Big stream" , "burn" .)