Spots knight in one (4)
I believe the answer is:
acne
'spots' is the definition.
(I've seen this before)
'knight in one' is the wordplay.
'knight' becomes 'n' (chess).
'in' is an insertion indicator.
'one' becomes 'ace' ('ace' can be a synonym of 'one'**).
'n' going into 'ace' is 'ACNE'.
(Other definitions for acne that I've seen before include "Skin rash" , "Skin problem, mainly of teenagers" , "sort of disease" , "Adolescent skin disease" , "Skin condition prevalent among teenagers" .)