Tail, then vanish out of sight (5)
I believe the answer is:
doggo
'out of sight' is the definition.
(synonyms)
'tail then vanish' is the wordplay.
'tail' becomes 'dog' (both can mean to follow).
'then' means one lot of letters go next to another.
'vanish' becomes 'go' (going is a kind of vanishing).
'dog'+'go'='DOGGO'
(Other definitions for doggo that I've seen before include "lying to avoid detection!" , "Quiet and hidden" , "(Lie) concealed" , "Quietly hidden" , "(Lie) silent and in hiding" .)