Registration work, mostly by chaps in hospital department (9)
I believe the answer is:
enrolment
'registration' is the definition.
(synonyms)
'work mostly by chaps in hospital department' is the wordplay.
'work mostly' becomes 'rol' (I can't explain this - if you can you should give a lot more credence to this answer).
'by' means one lot of letters go next to another.
'chaps' becomes 'men' (I've seen this before).
'in' is an insertion indicator.
'hospital department' becomes 'ENT'.
'rol'+'men'='rolmen'
'rolmen' put inside 'ent' is 'ENROLMENT'.
Can you help me to learn more?
(Other definitions for enrolment that I've seen before include "signing up" , "matriculation" , "Act of entering on a list of members" , "Registration" , "Joining" .)