Mediocre pub, almost run-down, one enters (10)
I believe the answer is:
suboptimal
'mediocre' is the definition.
Both the answer and definition are adjectives.
Perhaps you can see an association between them that I can't see?
'pub almost run-down one enters' is the wordplay.
'run-down' indicates anagramming the letters (a run-down or ruined version of the letters).
'one' becomes 'i' (Roman numeral).
'enters' means one lot of letters goes inside another.
'pub'+'almost'='pubalmost'
'pubalmost' is an anagram of 'suboptmal'.
'suboptmal' placed around 'i' is 'SUBOPTIMAL'.
Can you help me to learn more?
(Another definition for suboptimal that I've seen is " under par".)