Majority of gel pens in medical centre (6)
I believe the answer is:
clinic
'medical centre' is the definition.
(a clinic is a centre for medical treatment)
'majority of gel pens in' is the wordplay.
'majority of gel' becomes 'clic' (I can't explain this - if you can you should believe this answer much more).
'pens' means one lot of letters goes inside another (inserted letters are held or penned in).
'clic' enclosing 'in' is 'CLINIC'.
Can you help me to learn more?
(Other definitions for clinic that I've seen before include "Place or session at which treatment or advice is given" , "Private hospital" , "Healthcare facility" , "Medical resource" , "Place for specialised treatment or advice" .)