Gathering with a notable crowd once last bit of perimeter fence is fixed (5,10)
I believe the answer is:
press conference
'gathering with a notable' is the definition.
'crowd once last bit of perimeter fence is fixed' is the wordplay.
'crowd' becomes 'press' (press is a kind of crowd**).
'last bit of' says to take the final letters.
'is fixed' is an anagram indicator.
The final letter of 'perimeter' is 'r'.
'once'+'r'+'fence'='oncerfence'
'oncerfence' anagrammed gives 'conference'.
'press'+'conference'='PRESS CONFERENCE'
(Other definitions for press conference that I've seen before include "meeting with journalists" , "media event" , "Q&A session" , "media gathering" , "One way to get fruit juice" .)