Warn about following key muscle (5)
I believe the answer is:
brawn
'muscle' is the definition.
(I know that muscle can be written as brawn)
'warn about following key' is the wordplay.
'about' is an anagram indicator.
'following' means one lot of letters go next to another.
'key' becomes 'b' (eg B major, musical key).
'warn' is an anagram of 'rawn'.
'rawn' after 'b' is 'BRAWN'.
(Other definitions for brawn that I've seen before include "meatloaf" , "Physical strength; cooked meat" , "Jellied meat -- not brain!" , "Muscle; strength" , "Jellied meat - muscle" .)