Something loopy and irrational about religious books (5)
I believe the answer is:
picot
'something loopy' is the definition.
(I know this)
'irrational about religious books' is the wordplay.
'irrational' becomes 'pi' (an irrational number).
'about' becomes 'c' (circa).
'religious books' becomes 'OT' (Old Testament).
'pi'+'c'+'ot'='PICOT'
'and' is the link.
(Other definitions for picot that I've seen before include "lace work" , "loopy pattern" , "Topic about ornamental thread loop" , "Embroidery stitch - topic (anag)" , "Small embroidery loop" .)