Majority of gel pens in medical centre (6)
I believe the answer is:
clinic
This clue could be a double definition.
'majority of gel pens' is the first definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'medical centre' is the second definition.
(a clinic is a centre for medical treatment)
'in' is the link.
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" .)