Fastest part of UK motorway, optimum to go around 50 (8)
I believe the answer is:
nimblest
'fastest' is the definition.
Both the answer and definition are superlative adjectives. Perhaps you can see a link between them that I don't see?
'part of uk motorway optimum to go around 50' is the wordplay.
'part of uk' becomes 'NI' (Northern Ireland).
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'optimum' becomes 'best' (similar in meaning).
'to go around' indicates putting letters inside.
'50' becomes 'l' (Roman numeral).
'best' enclosing 'l' is 'blest'.
'ni'+'m'+'blest'='NIMBLEST'
Can you help me to learn more?
(Another definition for nimblest that I've seen is " Most agile, mentally of physically".)