Sally's first sweet potato (4)
I believe the answer is:
spud
'potato' is the definition.
(I've seen this before)
'sally's first sweet' is the wordplay.
'first' says to take the initial letters.
'sweet' becomes 'pud' (pud is a kind of sweet**).
The first letter of 'sallys' is 's'.
's'+'pud'='SPUD'
(Other definitions for spud that I've seen before include "Vegetable" , "One could be in jacket" , "one's eyed" , "Murphy" , "Tattie" .)