Almost register to secure popular health centre (6)
I believe the answer is:
clinic
'health centre' is the definition.
(I've seen this before)
'almost register to secure popular' is the wordplay.
I cannot quite understand how this works, but
'popular' could be 'in' (something that's 'in' is fashionable) and 'in' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for clinic that I've seen before include "Medical treatment centre" , "Outpatient session" , "Place for specialised treatment or advice" , "Medical resource" , "Building where outpatients are treated" .)