A mole that is infiltrating public relations (4)
I believe the answer is:
pier
'a mole' is the definition.
(thesaurus)
'that is infiltrating public relations' is the wordplay.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'infiltrating' is an insertion indicator.
'public relations' becomes 'pr'.
'ie' inserted into 'pr' is 'PIER'.
(Other definitions for pier that I've seen before include "Part of airport terminal; breakwater" , "Structure with a deck, built over water" , "Break-water or landing stage" , "A jetty or promenade" , "Platform out to sea" .)