Staple food like this needs a good filling (4)
I believe the answer is:
sago
'staple food' is the definition.
'sago' can be an answer for 'food' (I've seen this before). I am not sure about the 'staple' bit.
'this needs a good filling' is the wordplay.
'this needs' becomes 'so' (I am not sure about this - if you are sure you should believe this answer much more).
'a good' becomes 'ag' (I've seen this in other clues).
'filling' means one lot of letters goes inside another.
'so' going around 'ag' is 'SAGO'.
'like' is the link.
Can you help me to learn more?
(Other definitions for sago that I've seen before include "Starchy food as white pudding" , "Starchy grain used in desserts" , "Edible powdery starch" , "Powdery starch used in cooking" , "Edible palm starch" .)