After conclusion of diet concede being fat (6)
I believe the answer is:
tallow
'being fat' is the definition.
(fat used to make candles)
'after conclusion of diet concede' is the wordplay.
'after' is a charade indicator (letters next to each other).
'conclusion of' says to take the final letters.
'concede' becomes 'allow' (synonyms).
The final letter of 'diet' is 't'.
't' next to 'allow' is 'TALLOW'.
(Other definitions for tallow that I've seen before include "Fatty animal substance in soap making" , "Outstanding entertainer" , "burning fat" , "Candle ingredient" , "Fat processed for making candles" .)