Let in old boy, very English in old hat (8)
I believe the answer is:
obsolete
'old hat' is the definition.
(thesaurus)
'let in old boy very english' is the wordplay.
'in' means one lot of letters goes inside another.
'old boy' becomes 'ob'.
'very' becomes 'so' (I've seen this in other clues**).
'english' becomes 'e' (abbreviation).
'ob'+'so'+'e'='obsoe'
'let' going within 'obsoe' is 'OBSOLETE'.
'in' is the link.
(Other definitions for obsolete that I've seen before include "Pass" , "Out-of-date" , "Boots eel away, being out of date" , "Outdated" , "being out of practice" .)