Lively Spanish port run by old American (8)
I believe the answer is:
vigorous
'lively' is the definition.
(thesaurus)
'spanish port run by old american' is the wordplay.
'spanish port' becomes 'vigo' (I've seen this before).
'run' becomes 'r' (cricket abbreviation).
'by' says to put letters next to each other.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'american' becomes 'us'.
'vigo'+'r'+'o'+'us'='VIGOROUS'
(Other definitions for vigorous that I've seen before include "Energetic, strenuous" , "energetic,strenuous" , "Powerful, energetic" , "Energetic, forceful" , "Strong and energetic" .)