Subtly put a guard on a friend (9)
I believe the answer is:
gradually
'subtly put a' is the definition.
I can't tell whether this defines the answer.
'guard on a friend' is the wordplay.
I cannot really understand how this works, but
an anagram of 'guard' is 'gradu' which is present in the answer.
'friend' could be 'ally' (an ally is a friend or supporter) and 'ally' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for gradually that I've seen before include "A little at a time - slowly" , "over a period?" , "slow on the uptake?" , "slowly but surely" , "Bit by bit" .)