King entering battle with new cavalier, pessimist (3,7)
I believe the answer is:
wet blanket
'pessimist' is the definition.
(thesaurus)
'king entering battle with new cavalier' is the wordplay.
'king' becomes 'K' (abbreviation for king in chess).
'entering' is an insertion indicator.
'with' is a charade indicator (letters next to each other).
'cavalier' indicates an anagram (a cavalier or careless spelling).
'battle'+'new'='battlenew'
'battlenew' anagrammed gives 'wetblanet'.
'k' placed inside 'wetblanet' is 'WET BLANKET'.
(Other definitions for wet blanket that I've seen before include "Spoilsport" , "Person spoiling others' pleasure" , "what could moisten kippers" , "misery" , "Killjoy" .)