Eg Christmas fastener broken by a young relative (8)
I believe the answer is:
seasonal
'eg christmas' is the definition.
(as in the Christmas season)
'fastener broken by a young relative' is the wordplay.
'fastener' becomes 'seal' (seal is a kind of fastener).
'broken by' indicates putting letters inside (inserted letters break into the word).
'young relative' becomes 'son' (a child).
'a'+'son'='ason'
'seal' enclosing 'ason' is 'SEASONAL'.
(Other definitions for seasonal that I've seen before include "Varying according to time of year" , "Relating to spring, summer etc." , "Relating to a particular time of year" , "Like an Easter egg" , "specifically for Christmas period, say" .)