Pens, say - surprisingly integral part of communication network (7)
I believe the answer is:
synapse
'integral part of communication network' is the definition.
'pens say surprisingly' is the wordplay.
'surprisingly' is an anagram indicator.
'pens'+'say'='penssay'
'penssay' is an anagram of 'SYNAPSE'.
(Other definitions for synapse that I've seen before include "Where two nerve cells meet" , "Nerve junction" , "Part of the nervous system" , "Connection between nerve cells" , "Nerve cell connector" .)