One's left before in hospital department register (6)
I believe the answer is:
enlist
'register' is the definition.
(I've seen this before)
'one's left before in hospital department' is the wordplay.
'one's' becomes 'is' (roman numeral).
'left' becomes 'l' (common abbreviation**).
'before' says to put letters next to each other.
'in' indicates putting letters inside.
'hospital department' becomes 'ent'.
'is' after 'l' is 'lis'.
'lis' put within 'ent' is 'ENLIST'.
(Other definitions for enlist that I've seen before include "Obtain help of" , "Join up - tinsel (anag)" , "Register" , "Join the army" , "Listen (anag.)" .)