Aim, as before, to trap one (6)
I believe the answer is:
aspire
'aim' is the definition.
(synonyms)
'as before to trap one' is the wordplay.
'before' becomes 'pre' (prefix meaning 'before').
'to trap' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'pre' enclosing 'i' is 'pire'.
'as'+'pire'='ASPIRE'
(Other definitions for aspire that I've seen before include "rise to a great height" , "Long" , "Strive for higher things" , "Have a strong desire to reach an objective" , "Set your sights on" .)