Initiate opening, taking in new Hancock sketch (5,5)
I believe the answer is:
blood donor
'hancock sketch' is the definition.
(The Blood Donor is a sketch by Tony Hancock)
'initiate opening taking in new' is the wordplay.
'initiate' becomes 'blood' (blood can mean to initiate someone in an activity).
'opening' becomes 'door' (a door is an opening).
'taking in' is an insertion indicator.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'door' going around 'n' is 'donor'.
'blood'+'donor'='BLOOD-DONOR'
(Other definitions for blood donor that I've seen before include "One giving to NHS" , "Enabler of transfusion" , "whose contribution is vital" .)