Kernel of nut eaten by birds in New Testament letter (5)
I believe the answer is:
titus
'new testament letter' is the definition.
(Epistle of Paul to Titus)
'kernel of nut eaten by birds' is the wordplay.
'kernel of' indicates the central letters (kernel can mean something's core or centre).
'eaten by' indicates putting letters inside.
'birds' becomes 'tits' (tit is a kind of bird).
The middle of 'nut' is 'u'.
'u' going within 'tits' is 'TITUS'.
'in' is the link.
(Other definitions for titus that I've seen before include "- Andronicus (Shak.)" , "NT book" , "Rom. emperor 79 to 81AD" , "Only part of Shakespeare play" , "Groan, say" .)