Join up (5)
'join up' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for enrol that I've seen before include "Join up, sign on" , "Recruit" , "Add to list of members" , "Register (as member)" , "Register officially" .)
(Another definition for optin that I've seen is " Decide to take part".)
(Other definitions for connect that I've seen before include "Bring together" , "Join - link" , "Link together" .)
'join up' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for enlist that I've seen before include "Join the military" , "Join up - tinsel (anag)" , "Obtain help of" , "Take the Queen's shilling" , "Listen (anag.)" .)