Relent when accepting greeting that's late (6)
I believe the answer is:
behind
'that's late' is the definition.
(I've seen this before)
'relent when accepting greeting' is the wordplay.
'relent' becomes 'bend' (I've seen this in another clue).
'when accepting' indicates putting letters inside.
'greeting' becomes 'hi' (hi is a kind of greeting).
'bend' placed around 'hi' is 'BEHIND'.
(Other definitions for behind that I've seen before include "Not up to date" , "bum" , "Taking place in secret" , "in van? No" , "Seat" .)