Nuisance embracing single girl upset killjoy (9)
I believe the answer is:
pessimist
'killjoy' is the definition.
(thesaurus)
'nuisance embracing single girl upset' is the wordplay.
'nuisance' becomes 'pest' (I've seen this before).
'embracing' means one lot of letters goes inside another.
'single' becomes 'i' (one).
'girl' becomes 'miss' (I've seen this before**).
'upset' says the letters should be written in reverse.
'i'+'miss'='imiss'
'imiss' written backwards gives 'ssimi'.
'pest' going around 'ssimi' is 'PESSIMIST'.
(Other definitions for pessimist that I've seen before include "One expects the worst" , "The glass is always half-empty for this person" , "one anticipating the worst" , "Half-empty-glass person" , "One sees the glass as always being half-empty" .)