One undertaking study of the sun outside the church (7)
I believe the answer is:
scholar
'one undertaking study' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'the sun outside the church' is the wordplay.
'the sun' becomes 'solar' (I've seen this before).
'outside' means one lot of letters goes inside another.
'the church' becomes 'ch' (abbreviation on maps etc).
'solar' enclosing 'ch' is 'SCHOLAR'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for scholar that I've seen before include "Subject of poem by 1 across" , "Student or learned person" , "Learned type" , "Person of learning" , "university type?" .)