Sign up new clients first off (6)
I believe the answer is:
enlist
'sign up' is the definition.
(enlisting is a kind of signing up)
'new clients first off' is the wordplay.
'new' is an anagram indicator.
'first off' means to remove the first letter.
'clients' with its initial letter removed is 'lients'.
'lients' is an anagram of 'ENLIST'.
(Other definitions for enlist that I've seen before include "augment army" , "Obtain help of" , "Engage" , "Join up in silent way" , "Listen (anag.)" .)