Theme of a thousand old books and one poem (5)
I believe the answer is:
motif
'theme' is the definition.
(motif is a kind of theme)
'a thousand old books and one poem' is the wordplay.
'a thousand' becomes 'm' ('M' can be a synonym of 'thousand').
'old books' becomes 'ot' (Old Testament).
'and' means one lot of letters go next to another.
'one poem' becomes 'if' (I've seen this in other clues).
'm'+'ot'+'if'='MOTIF'
'of' acts as a link.
(Other definitions for motif that I've seen before include "Recurring image in design" , "Logo -- pattern" , "Recurring pattern" , "Shape" , "recurring idea" .)