At outset ambitious learner perhaps has aim to get top mark (5)
I believe the answer is:
alpha
'top mark' is the definition.
(an A-grade)
'at outset ambitious learner perhaps has aim' is the wordplay.
'at outset' suggests taking the first letters.
The first letters of 'ambitious learner perhaps has aim' is 'ALPHA'.
'to get' is the link.
(Other definitions for alpha that I've seen before include "First letter of the Greek alphabet" , "piece of Greek writing" , "First, best quality" , "- - and Omega" , "First in sequence" .)