In regret, neighbour turned up record (5)
I believe the answer is:
enter
'record' is the definition.
(enter can mean to enter into a record)
'in regret neighbour turned up' is the wordplay.
'in' says the answer is hidden in the clue.
'turned up' says the letters should be written backwards.
'retne' can be found hidden inside 'regret neighbour'.
'retne' in reverse letter order is 'ENTER'.
(Other definitions for enter that I've seen before include "Register (as a competitor)" , "Record (in a log)" , "Gain access to" , "Treen (anag.)" , "Go in - set down" .)