A great many you once observed beside coastal feature (9)
I believe the answer is:
thousands
'a great many' is the definition.
(many thousands of something)
'you once observed beside coastal feature' is the wordplay.
'you once' becomes 'thou' (historical second person pronoun).
'observed beside' means one lot of letters go next to another.
'coastal feature' becomes 'sands' (feature of a beach).
'thou'+'sands'='THOUSANDS'
(Other definitions for thousands that I've seen before include "Dan shouts out at many hundreds" , "1000s" , "Large numbers" , "a lot" , "Crowds" .)