Bird beginning to roost at home, enthralling old boy (5)
I believe the answer is:
robin
'bird' is the definition.
(robin is a kind of bird)
'beginning to roost at home enthralling old boy' is the wordplay.
'beginning to' indicates taking the first letters.
'at home' becomes 'in' ('I'm in' can mean 'I'm at home').
'enthralling' means one lot of letters goes inside another.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
The first letter of 'roost' is 'r'.
'r'+'in'='rin'
'rin' placed around 'ob' is 'ROBIN'.
(Other definitions for robin that I've seen before include "Small songbird" , "Red-breasted Christmas card bird" , ". . . . . Hood was a medieval outlaw" , "Seasonal bird" , "Batman and --" .)