Derelict old boy I see turning up (4)
I believe the answer is:
hobo
'derelict' is the definition.
(I've seen this in another clue)
'old boy i see turning up' is the wordplay.
'old boy' becomes 'ob' (abbreviation for a private school 'old boy').
'i see' becomes 'oh' (both can mean 'I understand').
'turning up' is a reversal indicator.
'ob'+'oh'='oboh'
'oboh' back-to-front is 'HOBO'.
(Other definitions for hobo that I've seen before include "Vagrant (American)" , "Vagrant, tramp" , "Tramp (US)" , "Bum" , "American tramp" .)