...joins more heartless aliens (5)
I believe the answer is:
meets
'joins' is the definition.
(meeting is a kind of joining)
'more heartless aliens' is the wordplay.
'heartless' means to remove the middle letters.
'aliens' becomes 'ETs' (ET is short for extraterrestrial).
'more' with its middle taken out is 'me'.
'me'+'ets'='MEETS'
(Other definitions for meets that I've seen before include "Joins with" , "Comes together with others" , "Comes into the presence of" , "sports gatherings" , "Comes into contact with" .)