Old queen's engaged trained nurse, one providing guarantees (7)
I believe the answer is:
ensurer
'one providing guarantees' is the definition.
The definition suggests a singular noun which matches the answer.
'old queen's engaged trained nurse' is the wordplay.
'old queen' becomes 're' (I can't justify this - if you can you should believe this answer much more).
'engaged' indicates putting letters inside (inserted letters are taken in or engaged).
'trained' indicates anagramming the letters (to train can mean to grow something into a shape).
'nurse' is an anagram of 'ensur'.
're' inserted into 'ensur' is 'ENSURER'.
Can you help me to learn more?
(Another definition for ensurer that I've seen is " One makes certain".)