Liar upset saint fast (9)
I believe the answer is:
fantasist
'liar' is the definition.
'upset saint fast' is the wordplay.
'upset' is an anagram indicator.
'saint'+'fast'='saintfast'
'saintfast' anagrammed gives 'FANTASIST'.
(Other definitions for fantasist that I've seen before include "Walter Mitty type?" , "Creator of very imaginative fiction" , "Dreamer" .)