Former ophthalmologist is in old religious group (7)
I believe the answer is:
oculist
'former ophthalmologist' is the definition.
(an older term for 'oculist')
'is in old religious group' is the wordplay.
'in' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'religious group' becomes 'cult' (I've seen this before).
'o'+'cult'='ocult'
'is' placed into 'ocult' is 'OCULIST'.
(Other definitions for oculist that I've seen before include "One working with seers" , "Old-fashioned eye doctor" , "Treater of eyes" , "One specialising in eye diseases" , "Person of vision" .)