Send back alien in right vessel (6)
I believe the answer is:
return
'send back' is the definition.
(thesaurus)
'alien in right vessel' is the wordplay.
'alien' becomes 'et' (extra terrestrial).
'in' is an insertion indicator.
'right' becomes 'r' (common abbreviation).
'vessel' becomes 'urn' (urn is a type of container).
'r'+'urn'='rurn'
'et' placed inside 'rurn' is 'RETURN'.
(Other definitions for return that I've seen before include "Two-way ticket" , "Send/come back" , "Key on computer" , "reply" , "Yield" .)