By securing a third off, there's reason to celebrate (8)
I believe the answer is:
birthday
'reason to celebrate' is the definition.
I can't tell whether this defines the answer.
'by securing a third off' is the wordplay.
'securing' is an insertion indicator.
'off' indicates anagramming the letters.
'a'+'third'='athird'
'athird' with letters rearranged gives 'irthda'.
'by' enclosing 'irthda' is 'BIRTHDAY'.
'there's' is the link.
Can you help me to learn more?
(Other definitions for birthday that I've seen before include "When presents are expected" , "Arrival time" , "when one was delivered" , "typically this suit amounts to nothing" , "Yearly anniversary" .)