Indiscretion in retro joint by fellow on ecstasy (5)
I believe the answer is:
gaffe
'indiscretion' is the definition.
(I've seen this before)
'retro joint by fellow on ecstasy' is the wordplay.
'retro' says the letters should be written backwards (retro is Latin for 'backwards').
'joint by' becomes 'fag' (I am not sure about this - if you are sure you should believe this answer much more).
'fellow' becomes 'f' (abbreviation - of a society etc.).
'on' means one lot of letters go next to another.
'ecstasy' becomes 'E' (informal term for the drug).
'fag' back-to-front is 'gaf'.
'gaf'+'f'+'e'='GAFFE'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for gaffe that I've seen before include "Blundering remark" , "Embarrassing blunder" , "embarrassing error" , "A foolish error" , "slip" .)