Enlist registered cure, improving it (7)
I believe the answer is:
recruit
'enlist' is the definition.
(I know that recruit is a more specific form of the action enlist)
'registered cure improving it' is the wordplay.
'registered' becomes 'R' (abbreviation, as in registered trademark).
'improving' indicates anagramming the letters (letters must be sorted out or rectified).
'cure' is an anagram of 'ecru'.
'r'+'ecru'+'it'='RECRUIT'
(Other definitions for recruit that I've seen before include "New entrant as to the army or police" , "New soldier" , "Take on" , "A new member or conscript" , "He's just joined up, often the armed forces" .)