Short flat sandwich needs new filling (4)
I believe the answer is:
snub
'short flat' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Maybe you can see a link between them that I can't see?
'sandwich needs new filling' is the wordplay.
'sandwich' becomes 'sub' (sub is a kind of sandwich).
'needs new' becomes 'n' (common abbreviation eg NT for New Testament. I am not sure about the 'needs' bit.).
'filling' means one lot of letters goes inside another.
'sub' placed around 'n' is 'SNUB'.
Can you help me to learn more?
(Other definitions for snub that I've seen before include "Act of rudeness" , "Insult; blunt (nose)" , "Refuse to acknowledge" , "Reject a person with cold behaviour" , "Reject greeting or approach" .)