Slowly changing the guard then mate follows (9)
I believe the answer is:
gradually
'slowly' is the definition.
(thesaurus)
'changing the guard then mate follows' is the wordplay.
'changing' indicates anagramming the letters.
'then mate' becomes 'ally' (I've seen this before. I am not sure about the 'then' bit.).
'follows' says to put letters next to each other.
'guard' with letters rearranged gives 'gradu'.
'gradu'+'ally'='GRADUALLY'
Can you help me to learn more?
(Other definitions for gradually that I've seen before include "A little at a time - slowly" , "slowly but surely" , "Bit by bit" , "Developing slowly and by degrees" , "slow on the uptake?" .)