Thingamy's party none enjoyed, on reflection (6)
I believe the answer is:
doodah
'thingamy's' is the definition.
(I've seen this before)
'party none enjoyed on reflection' is the wordplay.
'party' becomes 'do' (do can informally mean a party).
'none' becomes 'o'.
'enjoyed' becomes 'had' (I've seen this in other clues).
'on reflection' is a reversal indicator.
'had' in reverse letter order is 'dah'.
'do'+'o'+'dah'='DOODAH'
(Other definitions for doodah that I've seen before include "Thing whose name you can't remember" , "Thingummy" , "Object when someone forgets" , "Thingamy" , "one can't name" .)