Sergeants are led astray by elevation in rank (9)
I believe the answer is:
greatness
'elevation in rank' is the definition.
'greatness' can be an answer for 'rank' (thesaurus). I am not certain of the 'elevation in' bit.
'sergeants are led astray' is the wordplay.
'are led astray' indicates anagramming the letters.
'sergeants' is an anagram of 'GREATNESS'.
'by' is the link.
Can you help me to learn more?
(Other definitions for greatness that I've seen before include "Malvolio wasn't to fear being thrust into?" , "Nobility" , "''Some achieve ..., and some have ... thrust upon ?em'' (Twelfth Night)" , "Fame and importance" , "Pre-eminence; high station" .)