Old photograph is a peculiarly absorbing record (5)
I believe the answer is:
sepia
'old photograph' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'is a peculiarly absorbing record' is the wordplay.
'peculiarly' indicates anagramming the letters.
'absorbing' indicates putting letters inside.
'record' becomes 'EP' (abbreviation for extended play record).
'is'+'a'='isa'
'isa' with letters rearranged gives 'sia'.
'sia' going around 'ep' is 'SEPIA'.
Can you help me to learn more?
(Other definitions for sepia that I've seen before include "Former photographic pigment" , "Brown tint" , "Denizen of deep" , "Pigment obtained from cuttlefish ink" , "tone of pictures?" .)