The Spanish agent, potentially very smart (7)
I believe the answer is:
elegant
'smart' is the definition.
(similar in meaning)
'the spanish agent potentially' is the wordplay.
'the spanish' becomes 'el' ('the' in Spanish).
'potentially' indicates an anagram (letters have the potential to spell a new word).
'agent' is an anagram of 'egant'.
'el'+'egant'='ELEGANT'
'very' acts as a link.
(Other definitions for elegant that I've seen before include "Cleverly simple" , "Attractive and graceful or stylish" , "Gracefully stylish" , "Neat leg? (anag)" , "Modish" .)