Pongy during hard slog, one's taking time off (12)
I believe the answer is:
holidaymaker
'one's taking time off' is the definition.
'pongy during hard slog' is the wordplay.
'pongy' becomes 'olid' (both can mean extremely smelly).
'during' means one lot of letters goes inside another.
'hard slog' becomes 'haymaker' (term for a strong punch).
'olid' put into 'haymaker' is 'HOLIDAYMAKER'.
(Other definitions for holidaymaker that I've seen before include "Tourist" , "person in resort" , "taking a well-earned break?" .)