Former nun is retiring, becoming pensioner? (7)
I believe the answer is:
oldster
'pensioner?' is the definition.
(I've seen this before)
'former nun is retiring' is the wordplay.
'former' becomes 'old' (both can mean previous).
'nun' becomes 'rets' (I can't explain this - if you can you should believe this answer much more).
'is retiring' says the letters should be written in reverse.
'rets' back-to-front is 'ster'.
'old'+'ster'='OLDSTER'
'becoming' acts as a link.
Can you help me to learn more?
(Other definitions for oldster that I've seen before include "Senior citizen (mainly US)" , "One aged" , "Experienced person" , "one getting on" , "One no longer inexperienced" .)