Dear girl, come back in for a little sustenance (6)
I believe the answer is:
peanut
'a little sustenance' is the definition.
The definition and answer can be both things that one consumes for nourishment as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'dear girl come back in' is the wordplay.
'dear' becomes 'pet' (I've seen this in other clues).
'girl' becomes 'una'.
'come back' is a reversal indicator.
'in' is an insertion indicator.
'una' reversed gives 'anu'.
'pet' enclosing 'anu' is 'PEANUT'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for peanut that I've seen before include "Salted or dry roasted item" , "It ripens in a vine pod underground" , "(Plant with) edible underground seed" , "Singularly little money" , "Dry-roasted or salted snack item" .)