Aim, as before, to trap one (6)
I believe the answer is:
aspire
'aim' is the definition.
(I know that aim can be written as aspire)
'as before to trap one' is the wordplay.
'before' becomes 'pre' (prefix meaning 'before').
'to trap' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'pre' placed around '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" .)