Put it back on, you old monster! (4)
I believe the answer is:
yeti
'monster' is the definition.
(I know that yeti is a type of legendary creature)
'put it back on you old' is the wordplay.
'put it back' becomes 'ti' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'on' is a charade indicator (letters next to each other).
'you old' becomes 'ye' (historical version of 'you').
'ti' after 'ye' is 'YETI'.
Can you help me to learn more?
(Other definitions for yeti that I've seen before include "Mysterious Himalayan creature" , "Legendary snowman" , "doubts in its existence" , "elusive character" , "Himalayan legend" .)