Stab knight during hiatus in retreat (4)
I believe the answer is:
pang
'stab' is the definition.
(I have seen 'Stab of emotion ' mean 'pang' so perhaps 'stab' could also mean 'pang')
'knight during hiatus in retreat' is the wordplay.
'knight' becomes 'n' (chess).
'during' is an insertion indicator.
'hiatus' becomes 'gap' (I've seen this before).
'in retreat' says the letters should be written in reverse.
'n' placed inside 'gap' is 'gnap'.
'gnap' in reverse letter order is 'PANG'.
(Other definitions for pang that I've seen before include "Sharp emotional pain" , "Stab of emotion" , "Brief pain" , "Stab of pain" , "Short violent pain" .)