In bathtub lurks a smudge (4)
I believe the answer is:
blur
'smudge' is the definition.
('blur' can be a synonym of 'smudge')
'in bathtub lurks a' is the wordplay.
'in' says the answer is hidden in the clue.
'BLUR' can be found hidden inside 'bathtub lurks a'.
(Other definitions for blur that I've seen before include "Out-of-focus image" , "Out-of-focus sight" , "Unfocused image" , "Hazy patch" , "Make vague" .)