Mentioned having seen (8)
I believe the answer is:
observed
I believe this clue is a double definition.
'mentioned' is the first definition.
('observe' can be a synonym of 'mention')
'seen' is the second definition.
(synonyms)
'having' is the link.
(Other definitions for observed that I've seen before include "Ascertained by sight" , "Seen" , "saw" , "Noticed or abided by" , "Abided by, rules say" .)