Chapter on old Irish group of singers (5)
I believe the answer is:
choir
'group of singers' is the definition.
(a choir is a singing group)
'chapter on old irish' is the wordplay.
'chapter' becomes 'ch'.
'on' says to put letters next to each other.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'irish' becomes 'IR' (abbreviation).
'ch'+'o'+'ir'='CHOIR'
(Other definitions for choir that I've seen before include "Carol singers?" , "Organised group of singers" , "warblers" , "Vocal group" , "Body of singers" .)