Perfect declaration of one involved in trade (5)
I believe the answer is:
ideal
'perfect' is the definition.
(similar in meaning)
'declaration of one involved in trade' is the wordplay.
'declaration of one involved in' becomes 'i' (I am not sure about this - if you are sure you should believe this answer much more).
'trade' becomes 'deal' (synonyms).
'i'+'deal'='IDEAL'
(Other definitions for ideal that I've seen before include "that can't be bettered?" , "Model of perfection" , "Coal-mining waste" , "Model of excellance" , "Just perfect, couldn't be better" .)