Puzzle’s fine, can decipher at last (5)
I believe the answer is:
floor
'puzzle's' is the definition.
(I've seen this before)
'fine can decipher at last' is the wordplay.
'fine' becomes 'f' (abbreviation).
'can' becomes 'loo' (slang).
'at last' indicates one should take the final letters.
The last letter of 'decipher' is 'r'.
'f'+'loo'+'r'='FLOOR'
(Other definitions for floor that I've seen before include "Disconcert" , "Knock down; storey" , "Bottom surface" , "Base of room - storey" , "Lower limit" .)