Identify communication system to return missing link (6)
I believe the answer is:
apeman
'missing link' is the definition.
(missing link in human evolution)
'identify communication system to return' is the wordplay.
'identify' becomes 'name' (synonyms).
'communication system' becomes 'pa' (PA is an example).
'to return' says the letters should be written backwards.
'name'+'pa'='namepa'
'namepa' in reverse letter order is 'APEMAN'.
(Other definitions for apeman that I've seen before include "Long extinct human ancestor" , "Extinct primate" , "Primitive figure" , "Missing link, possibly" , "One of our extinct ancestors?" .)