Mum's mum having nothing to eat: complain (5)
I believe the answer is:
groan
'complain' is the definition.
(I've seen this before)
'mum's mum having nothing' is the wordplay.
'mum's mum' becomes 'gran' (I've seen this before).
'having' means one lot of letters goes inside another.
'nothing' becomes 'O' (looks like zero - 0).
'gran' enclosing 'o' is 'GROAN'.
'to eat' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for groan that I've seen before include "Utterance expressing pain or disapproval" , "Complain" , "Complaint" , "Organ (anag) - lament" , "Titus -- (Mervyn Peake)" .)