Long-winded poem about retired old boy (7)
I believe the answer is:
verbose
'long-winded' is the definition.
(long-winded can mean overly talkative or verbose)
'poem about retired old boy' is the wordplay.
'poem' becomes 'verse' (verse is a kind of poem).
'about' indicates putting letters inside.
'retired' shows that the letters should be reversed in order.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'ob' backwards is 'bo'.
'verse' placed around 'bo' is 'VERBOSE'.
(Other definitions for verbose that I've seen before include "Using more words than necessary" , "Loquacious" , "Boringly long-winded" , "Circumlocutory" , "'Long-winded, wordy (7)'" .)