Have ambition and, like before, I enter (6)
I believe the answer is:
aspire
'have ambition' is the definition.
(I've seen this in another clue)
'like before i enter' is the wordplay.
'like' becomes 'as'.
'before' becomes 'pre' (prefix meaning 'before').
'enter' means one lot of letters goes inside another.
'pre' enclosing 'i' is 'pire'.
'as'+'pire'='ASPIRE'
'and' acts as a link.
(Other definitions for aspire that I've seen before include "rise to a great height" , "Have aspiration for, aim at" , "Aim, have an ambitious plan" , "Long" , "Aim, shoot for" .)