Colour of holy chap holding Mass (7)
I believe the answer is:
pigment
'colour' is the definition.
(I've seen this before)
'holy chap holding mass' is the wordplay.
'holy' becomes 'pi' (I've seen this in other clues).
'chap' becomes 'gent' (I've seen this in other clues**).
'holding' is an insertion indicator.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'gent' going around 'm' is 'gment'.
'pi'+'gment'='PIGMENT'
'of' is the link.
(Other definitions for pigment that I've seen before include "Colour or dye" , "Colouring material" , "Colouring substance" , "Colouring matter" , "Colourful substance" .)