Send it back after the old monster (4)
I believe the answer is:
yeti
'monster' is the definition.
(yeti is a kind of monster)
'send it back after the old' is the wordplay.
I cannot quite see how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
'the old' could be 'ye' (resembles a historical spelling of 'the') and 'ye' is found in the answer.
A single letter 'i' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for yeti that I've seen before include "Himalayan legend" , "A sort not reliably recorded" , "Highly elusive monster" , "Hairy Himalayan humanoid" , "that you might find in mountains" .)