Metrical patterns in book one friend abroad carries (5)
I believe the answer is:
iambi
'metrical patterns' is the definition.
(I've seen this before)
'book one friend abroad carries' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'one' becomes 'i' (Roman numeral).
'friend abroad' becomes 'ami' (I've seen this in another clue).
'carries' is an insertion indicator.
'i'+'ami'='iami'
'b' put inside 'iami' is 'IAMBI'.
'in' acts as a link.
(Other definitions for iambi that I've seen before include "Metrical units with alternating long and short syllables" , "Metrical patterns" , "Feet of two syllables (one short and one long)" , "Short-long metrical feet" , "Poetic feet" .)