Content to leave Eton, learner first to join up (6)
I believe the answer is:
enlist
'join up' is the definition.
(I've seen this in another clue)
'content to leave eton learner first' is the wordplay.
'content to leave' means to remove the middle letters (the contents of the word are removed).
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'first' becomes 'ist' (ist resembles 1st).
'eton' with its middle removed is 'en'.
'en'+'l'+'ist'='ENLIST'
'to' acts as a link.
(Other definitions for enlist that I've seen before include "Join the armed services" , "Register" , "Join up in silent way" , "Listen (anag.)" , "Obtain help of" .)