Son remains in frame (4)
I believe the answer is:
sash
'frame' is the definition.
(I've seen this before)
'son remains' is the wordplay.
'son' becomes 's'.
'remains' becomes 'ash' (I've seen this before).
's'+'ash'='SASH'
'in' acts as a link.
(Other definitions for sash that I've seen before include "Type of sliding window" , "Window frame - shoulder band" , "Part of a window - decoration" , "Band - scarf" , "Waistband - type of window" .)