Colour encapsulated by Fauvist in theory (4)
I believe the answer is:
tint
'colour' is the definition.
(tint is a kind of colour)
'encapsulated by fauvist in theory' is the wordplay.
'encapsulated by' says the answer is hidden in the clue (the clue encapsulates or contains the answer).
'TINT' can be found hidden inside 'fauvist in theory'.
(Other definitions for tint that I've seen before include "work for coiffeuse?" , "(Softer variety of) colour" , "A slight shade of colour" , "Hue, shade" , "'Shade, tone (4)'" .)