Tail to disappear out of sight (5)
I believe the answer is:
doggo
'out of sight' is the definition.
('doggo' can be a synonym of 'out of sight')
'tail to disappear' is the wordplay.
'tail' becomes 'dog' ('dog' can be a synonym of 'tail').
'to disappear' becomes 'go' (going is a kind of disappearing).
'dog'+'go'='DOGGO'
(Other definitions for doggo that I've seen before include "neither seen nor heard" , "keeping mum out of the picture?" , "Lie . . . . . , motionless or hidden" , "(Lie) silent and in hiding" , "in secret" .)