Fleeting record, fellow rebuilding realm (9)
I believe the answer is:
ephemeral
'fleeting' is the definition.
(I've seen this in another clue)
'record fellow rebuilding realm' is the wordplay.
'record' becomes 'EP' (abbreviation for extended play record).
'fellow' becomes 'he' ('he' refers to a man).
'rebuilding' indicates an anagram.
'realm' with letters rearranged gives 'meral'.
'ep'+'he'+'meral'='EPHEMERAL'
(Other definitions for ephemeral that I've seen before include "just for a day?" , "Transitory" , "Short lived" , "not going to last long" , "short term gain" .)