Start short date with posh fellow (3,3)
I believe the answer is:
set off
'start' is the definition.
(I know that start can be written as set off)
'short date with posh fellow' is the wordplay.
'short' means to remove the last letter.
'date' becomes 'see' (synonyms).
'with posh fellow' becomes 'toff' (I am not sure about this - if you are sure you should believe this answer much more).
'see' with its last letter taken away is 'se'.
'se'+'toff'='SET-OFF'
Can you help me to learn more?
(Other definitions for set off that I've seen before include "'Leave on a journey (3,3)'" , "Begin journey or cause to explode" , "contrast" , "Counterbalance" , "Display - mount - start journey" .)