Yours truly’s up for eating a potato (3)
I believe the answer is:
yam
'potato' is the definition.
(I have seen 'Sweet potato' mean 'yam' so perhaps 'potato' could also mean 'yam')
'yours truly's up for eating a' is the wordplay.
'yours truly's' becomes 'my' (belonging to the person writing or speaking).
'up' is a reversal indicator (in down clue: letters go upwards).
'for eating' means one lot of letters goes inside another.
'my' backwards is 'ym'.
'ym' placed around 'a' is 'YAM'.
(Other definitions for yam that I've seen before include "US sweet potato" , "Edible root" , "Tuberous vegetable" , "American sweet potato" , "May be a sweet potato" .)