Went running back when late (4)
I believe the answer is:
deep
'late' is the definition.
('deep' can be a synonym of 'late')
'went running back' is the wordplay.
'went' becomes 'peed' (both can mean urinated).
'running back' shows that the letters should be reversed in order.
'peed' written backwards gives 'DEEP'.
'when' acts as a link.
(Other definitions for deep that I've seen before include "Intense -- extreme" , "(Of sound) low in pitch" , "Extending far down" , "greatly involved?" , "Mysterious" .)