Thanks for mashing the rest of the spuds (6)
I believe the answer is:
taters
'the spuds' is the definition.
('tater' can be a synonym of 'spud')
'thanks for mashing the rest' is the wordplay.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'for' means one lot of letters go next to another.
'mashing' is an anagram indicator.
'rest' with letters rearranged gives 'ters'.
'ta'+'ters'='TATERS'
'of' acts as a link.
(Other definitions for taters that I've seen before include "Potatoes (dialect)" , "starchy tubers" .)