Smallest final involving England’s capital (5)
I believe the answer is:
least
'smallest' is the definition.
(synonyms)
'final involving england's capital' is the wordplay.
'final' becomes 'last' (synonyms).
'involving' means one lot of letters goes inside another.
'capital' suggests taking the first letters.
The initial letter of 'englands' is 'e'.
'last' going around 'e' is 'LEAST'.
(Other definitions for least that I've seen before include "Superlatively little" , "Smallest (in significance)" , "superlatively small" , "Lowest amount" , "To the lowest degree" .)