Old Greek colony with aliens on it, periodically returning (5)
I believe the answer is:
ionia
'old greek colony' is the definition.
(I've seen this in another clue)
'aliens on it periodically returning' is the wordplay.
'periodically' indicates alternate letters (letters must be selected 'every so often').
'returning' shows that the letters should be reversed in order.
'aliens'+'on'+'it'='aliensonit'
The alternating letters of 'aliensonit' are 'ainoi'.
'ainoi' in reverse letter order is 'IONIA'.
'with' acts as a link.
(Other definitions for ionia that I've seen before include "where Greeks settled" , "Old territory" , "Greek area of Asia Minor" , "part of Asia Minor" , "Ancient area" .)