Measure introduced by country, spreading from the centre (8)
I believe the answer is:
stellate
'spreading from the centre' is the definition.
Both the answer and definition are adjectives. Maybe you can see a link between them that I can't see?
'measure introduced by country' is the wordplay.
'measure' becomes 'ell' (historical unit of length).
'introduced by' is an insertion indicator (some letters introduce others into them).
'country' becomes 'state' (synonyms).
'ell' going within 'state' is 'STELLATE'.
Can you help me to learn more?
(Another definition for stellate that I've seen is " making five points, typically".)