Discourage injured Rooney dropping back at World Cup (4,4,5,2)
I believe the answer is:
pour cold water on
'discourage' is the definition.
(I've seen this in another clue)
'injured rooney dropping back at world cup' is the wordplay.
'injured' indicates anagramming the letters.
'dropping back' means to remove the last letter.
'rooney' with its final letter taken away is 'roone'.
'roone'+'at'+'world'+'cup'='rooneatworldcup'
'rooneatworldcup' anagrammed gives 'POUR COLD WATER ON'.
(Other definitions for pour cold water on that I've seen before include "Discourage" , "dangerous way to put out chip-pan fire" .)