Ruin one strange gathering of old friends (7)
I believe the answer is:
reunion
'gathering of old friends' is the definition.
(I've seen this before)
'ruin one strange' is the wordplay.
'strange' indicates an anagram.
'ruin'+'one'='ruinone'
'ruinone' anagrammed gives 'REUNION'.
(Other definitions for reunion that I've seen before include "Gathering of old friends, colleagues etc" , "Meet-up of old friends" , "Eg, old boys' gathering" , "French island near Mauritius" , "Get-together after some time apart" .)