Every MP has one small worry (4)
I believe the answer is:
seat
'every mp' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'one small worry' is the wordplay.
'one small' becomes 's' (abbreviation - e.g. clothes size).
'worry' becomes 'eat' (something worrying may eat someone up).
's'+'eat'='SEAT'
'has' acts as a link.
Can you help me to learn more?
(Other definitions for seat that I've seen before include "Chair, bench, or stool, say" , "A manner of sitting on horse" , "Spanish car" , "Parliamentary constituency" , "Settle in" .)