Make later improvements to crumbling outer surfaces of church (7)
I believe the answer is:
Here is my best explanation:
'make later improvements' is the definition.
'crumbling outer surfaces of church' is the subsidiary indication.
'crumbling' indicates an anagram.
'surfaces of' says to hollow out the word (remove centre letters).
'church' with its centre removed is 'ch'.
'outer' is an anagram of 'retou'.
'to' is the link.
(Other definitions for retouch that I've seen before include "Make slight improvements to (a photo/picture)", "Paint over", "Improve a photo, with a pencil say" and "Repair a painting".)