River runs through marquee (5)
I believe the answer is:
trent
'river' is the definition.
(River Trent)
'runs through marquee' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'through' indicates putting letters inside (inserted letters go through the word).
'marquee' becomes 'tent' (I've seen this in other clues).
'r' going into 'tent' is 'TRENT'.
(Other definitions for trent that I've seen before include "Crossword frame" , "running water" , "Nottingham's river" , "River through Newark" , "Major English river" .)