Troops slip over wet blanket (10)
I believe the answer is:
spoilsport
'wet blanket' is the definition.
('spoilsport' can be a synonym of 'wet blanket')
'troops slip over' is the wordplay.
'over' is an anagram indicator.
'troops'+'slip'='troopsslip'
'troopsslip' anagrammed gives 'SPOILSPORT'.
(Other definitions for spoilsport that I've seen before include "misery" , "Wet blanket" , "Damper" , "Party pooper" , "One who won't play?" .)