One with a dream lover posed from behind first (9)
I believe the answer is:
fantasist
'one with a dream' is the definition.
(someone who fantasises about something)
'lover posed from behind first' is the wordplay.
'lover' becomes 'fan' (synonyms).
'posed' becomes 'sat' ('sit' can be a synonym of 'pose').
'from behind' says the letters should be written backwards.
'first' becomes 'ist' (ist resembles 1st).
'sat' back-to-front is 'tas'.
'fan'+'tas'+'ist'='FANTASIST'
(Other definitions for fantasist that I've seen before include "Creator of very imaginative fiction" , "Dreamer" , "Walter Mitty type?" .)