Give diet up, having eaten last of this? (9)
I believe the answer is:
digestive
'give diet up having eaten last of this?' is the definition.
The definition suggests a singular noun but the answer is not.
'give diet up having eaten last of this?' is the wordplay.
'up' is an anagram indicator (letters get chucked up).
'having eaten' is an insertion indicator.
'last of' suggests the final letters.
The final letter of 'this' is 's'.
'give'+'diet'='givediet'
'givediet' is an anagram of 'digetive'.
'digetive' going around 's' is 'DIGESTIVE'.
(Another definition for digestive that I've seen is " Type of biscuit".)